Our morphological comparisons show that in many cases we can unambiguously identify a pleotelson, a true telson, and of course, directly observe the location of the anus. Problems arise when it appears that the telson is completely fused to a sixth pleomere bearing a terminal anus and terminal uropods. In the latter case, we are unable to determine whether the telson was ever present at all. Only a consideration of development stages can provide information to answer that question.
All Pera- and Pancarida possess a direct mode of development. In both groups,most of the post-cephalic segments originate from teloblasts (ectoteloblasts and mesoteloblasts), which bud off their descendant cells in an anterior to posterior direction (Richter and Scholtz, 2001). In this process, a grid like pattern forms on the germ band of peracarids. However, as noted earlier, the amphipods lack ectoteloblasts. In this respect, amphipods are very special. Nevertheless, they show the mentioned grid-like pattern of longitudinal and transversal cell rows in the embryo (Scholtz 1990, 2000).
The proctodaeum can occur in a variety of places. In most species of peracarids investigated so far, the proctodaeum is initiated a short distance behind the row of ectoteloblasts, e.g., two to three cell rows behind the ectoteloblasts in Hemimysis lamornae (Manton, 1928; Nair, 1939) and in several species of Isopoda (Nair, 1956; McMurrich, 1895; Strömberg, 1972). In amphipods, the initial proctodaeal invagination occurs several cell rows behind those of the last pleonal segment (Weygoldt, 1958, Ungerer and Wolff, 2005). In tanaidaceans (Scholl, 1963) and the isopod Idotea (Strömberg, 1965), the anus forms immediately behind the teloblasts. In contrast, the cumaceans and Thermosbaena mirabilis offer a quite extended area of cell proliferation behind the teloblasts, and the proctodaeum appears at the very tip of this region (Dohrn, 1870; Zilch, 1974, 1975).
The telson in Recent Tanaidacea is probably completely reduced. In his investigations concerning the development of Heterotanais oerstedi, Scholl (1963) recognized a peculiarity concerning the structure of the forming proctodaeum. He indicated that the proctodaeum of tanaidaceans, which forms immediately behind the ectoteloblasts, drags many cells into the interior of the developing embryo. Before the formation of the proctodaeum started, these cells were located posterior to the area of invagination. In other words, these are cells of the putative telson Anlage in the early embryo. During the ontogeny of the anus, these cells are involved in hindgut formation and therefore do not contribute to the formation of a telson of the adult. In contrast to the dorsal wall of the forming proctodaeum, the ventral wall of the tanaidacean anus seems to form from only few (blastoderm) cells that divide repeatedly. Because of this, the ventral side of the proctodaeum features a different type of cell. The described process might correspond to a reduction in size of the posterior-most part of the germ band (telson Anlage), and the process might depict a loss of the adult telson. Unfortunately, Scholl did not discuss comprehensively the fate of the ectoderm that is located posterior to the ectoteloblasts.
We encounter a special case in Orchestia cavimana. The pleomeres are initially specified by a grid-like alignment of ectodermal cells (Fig. 9.1; Ungerer and Wolff, 2005), which is especially evident posteriorly. In this case, clear delineation of seven segmental units can be detected, and a little later, an eighth unit marks the terminus of the germ band (Fig. 9.3). We observed that the proctodaeal invagination occurs in this eighth unit (Fig. 9.4), which might then be identified as a putative telson. However, shortly after the proctodaeum begins to invaginate, two latero-ventral swellings begin to form (Fig. 9.5) on either side of the putative anus. As development progresses, the distinctness of the tissues of the eighth unit fades as the anus and paired lobes swell, and the seventh pleomere unit appears to shrink. Consequently, the anus comes to open on the posterior aspect of the sixth pleomere. Meanwhile, the separate paired lobes (Fig. 9.5) continue to enlarge, project posteriorly, and migrate to a dorsal position (Fig. 9.6) relative to the anus. In Orchestia cavimana , these two lobes remain separated and form what is generally termed the cleft telson. Our concurrent observation of the development of Hyalella azteca , reveals a similar sequence of events, except in this species the paired lobes come together in the area dorsal to the anus and fuse to form a single shelf-like structure.
It would appear, from our interpretation, that the structure we commonly refer to as a telson in amphipods in fact develops rather differently from those telsons we encounter in other groups of malacostracans such as the mysidaceans (see discussion below). The amphipod lobes seem to be akin to caudal rami in their manner of appearance.