788 MR W. T. CALMAN ON 



thoracic " segment from the head, does not correspond to a movable articulation like 

 those which separate the other segments of the body, but to a superficial groove in the 

 integument (Plate I. fig. 2, c). That this groove does not represent the line of 

 junction of the first thoracic segment with the head is, we think, shown by the 

 fact that, instead of passing directly downwards on each side, parallel to the hinder 

 margin of the segment, it runs obliquely forwards and ends just behind the root of 

 the mandible, so that between it and the posterior border of the segment is 

 included the region of the two pairs of maxillae in addition to that of the maxillipeds 

 (Plate I. fig. 1). Mr Thomson's "first thoracic segment" must represent, therefore, 

 the fused segments corresponding to these three pairs of appendages. Further, 

 the position of this groove is precisely that of the " cervical sulcus," which, in the 

 Mysidse, crosses the carapace immediately above the mandibles, and with this sulcus 

 it must, we think, be identified. And, as there is no reason to suppose that the 

 cervical sulcus of the Mysidse is other than the representative of the cervical groove 

 of the Decapods, we are led to the presumption that the segments bearing the two 

 pairs of maxillae are morphologically posterior to this groove in the Decapods also, a 

 presumption against which no anatomical evidence appears to militate. And if all 

 this be true, we should then perceive the existence alike in Anaspides, in the 

 Mysidse, and in the Decapods, of a primary sulcus delimiting an anterior region or 

 head to which three pairs of appendages, antennules, antennae, and mandibles belong, 

 the region in fact of the three paired appendages of the Nauplius, the " primary head 

 region" of the Crustacea according to Claus. # 



From this cervical groove in Anaspides there runs back on each side a horizontal 

 line (fig. 2, a) which marks off inferiorly a quadrilateral area. Just behind the cervical 

 groove a faint impressed line can be traced (fig. 2, b), for the most part nearly parallel 

 with the main groove, but bending backwards for a short distance in the dorsal region, 

 and terminating on each side near the junction of the horizontal line with the cervical 

 groove. 



While unable to suggest any definite interpretation of these various markings, we 

 may note that Mr Thomson's comparison of the horizontal line with that which marks 

 off the " epimeron " on the thoracic segments of various Isopoda seems to be quite 

 untenable. The " epimeron" in that group represents the coxal joint of the leg, and it 

 is never marked off on the first thoracic segment, which in the Isopoda is indistinguish- 

 ably fused with the head. 



On the dorsal surface of the cephalic region, a short distance in front of the cervical 

 groove, a small ill-defined area is more darkly pigmented than the surrounding parts, 

 appearing to the naked eye as a distinct dark brown spot. In the centre of this area 

 there is a' clear, retractile, circular spot, surrounded by four smaller dots equidistant 



* This suggestion deals with the morphology of the carapace only in so far that it leaves out of account the 

 many complex issues that might arise from a full consideration of the various minor grooves, other than the main 

 cervical one, which are described by Boas (Dccaj). Slaegtskab., Vidensk. Selslc. Shr., 1880) as present on the carapace 

 of various Decapods. 



