THE CRUSTACEA 



General Morphology. 



Exoskeleton. — In the Crustacea, as in other Arthropoda, the exo- 

 skeleton of the body consists typically of a series of segments or 

 somites which may be movably articulated or more or less coalesced 

 together. In its simplest form the exoskeletal somite is a ring of 

 chitiu, connected with the adjacent rings by areas of thinner 

 integument permitting movement in various directions, and having 

 a pair of appendages attached to its ventral surface. This ring may 

 be further subdivided into a dorsal tergite and a ventral sternite, and 



the tergite may overhang the 

 attachment of the appendage on 

 each side as a free plate called the 

 plewon ^ (Fig. 1). 



At the posterior end of the 

 body is a terminal segment known 

 as the telson, upon which the anus 

 opens. This segment never bears 

 typical limbs and its nature has 

 been variously interpreted. It has 

 been regarded as a true somite or 

 as resulting from the coalescence 

 of a number of somites, while some 

 have described it as a "median 

 appendage" or as a fused pair of appendages. Its true nature, 

 however, is clearly shown by embryology. In the larval develop- 

 ment of the more primitive Crustacea the body increases in length 

 by the successive addition of new somites between the last-formed 

 somite and the terminal region Avhich bears the anus. The " 2;row- 

 ing-point " is, in fact, situated in front of this region, and when 

 the fidl number of somites has been reached, the unsegmented part 

 remaining forms the telson of the adult. 



In no Crustacean, however, do all the somites of the body 

 remain distinct. Coalescence of somites or suppression of segmenta- 

 tion (lipomerism) involves more or less extensive regions where the 

 component somites are only indicated by the persistence of the 

 corresponding appendages. This is constantly the case in the 

 anterior part of the body, where a varying number of somites are 

 united to form the head. This fusion of cephalic somites is 

 associated with what Lankester has termed the " adaptational shift- 

 ing of the oral aperture " backwards from its primitive position at 

 the anterior end of the body. As a result of this shifting, at least two 

 somites, corresponding to the antennules and antennae, come to lie 



P- 



A transverse section of one of tlie 

 abdominal somites of the Crayfish (A!<farus). 

 a, appendage ; p, pleuron ; n, sternite ; t, 

 tergite. 



1 Sonietinic's called the cjdmero)!, but this term has been used in diB'ereiit senses 

 and it seems better to abandon it altogether. 



