360 INVERTEBBATE MORPHOLOGT. 



on each side of the head, the fourth, counting from the ante- 

 rior mid-line, being longer than the rest, usually destitute of 

 suckers except towards the tip, and in most species kept 

 retracted within a groove on each side of the head except 

 when required for prehension. They are all good swimmers, 

 and the body is elongated and provided with lateral fins of 

 greater or less extent. 



A shell is present in the Decapod a, but shows a great 

 reduction in size and complexity from that of the Tetra- 

 branchiates ; and in order to understand its homologies in the 

 diiferent genera it will be necessary to obtain an idea of its 

 form in the fossil members of the group which occur in the 

 Mesozoic rocks forming the family Belemnitidse. In the 

 genus Belemnites (Fig. 161, B), for instance, the shell consisted 

 of a terminal conical solid portion, termed the rostrum (r), 

 the base of which was hollow and contained a chambered 

 shell, the phragmacone (pA), corresponding to the Nautilus 

 shell, the anterior portion of the last chamber of this being 

 elongated into a broad flat process termed the proostracon 

 {pr). By various modifications of this structure the shells of 

 the different living Decapods have been developed. In Spirida 

 the shell is coiled into a spiral and is partly enclosed by the 

 mantle, the rostral and proostracal portions having disap- 

 peared. In all other forms the shell has a more or less flat- 

 tened form and becomes completely enclosed within the 

 mantle, folds of which grow up around it. In Sepia (Fig. 

 161, A) the proostracon becomes almost obliterated and the 

 rostrum (r) is exceedingly small, the phragmacone {ph) form- 

 ing the principal bulk of the shell. This, however, has become 

 very much modified — that portion of it which lies posterior to 

 the sipuncle (s) ceasing to develop, or rather becoming exceed- 

 ingly compact by the various partitions lying in close con- 

 tact with one another without any intervening air-chambers. 

 These chambers are, however, developed in the portion ante- 

 rior to the sipuncle, but are comparatively flat and traversed 

 by calcareous spicules, so that the shell has a somewhat 

 spongy appearance and is exceedingly light. In other forms, 

 however, the proostracon is the portion that persists, the 

 rostrum and phragmacone both disappearing (Fig. 161, G), so 



