viii THE VOYAGE OF H.M.S. CHALLENGER. 



pereionic, and it would therefore appear that there must be five somites (A to E) only 

 'belonging to the first division or cephalon, nine (F to 0) to the second or pereion, and 

 «even (P to Z) to the third or pleon. 



In the mature forms the encroachment of one part on the other is so marked and 

 conspicuous .that several of the more crowded appendages lose their simple character and 

 adapt themselves to the functions of those with which they are brought into closer affinity ; 

 thus the anterior pairs of pereiopoda, which are true feet in the simple forms, become 

 hands, and then in still closer resemblance to the oral appendages, until in the more 

 highly developed forms the second pair of gnathopoda loses its pediform character and 

 becomes in the Brachyura little more than opercula, covering the mouth. Another 

 fact brought out in the study of these and other immature forms during the progress 

 of their development is that the carapace is structurally independent of the pereion, 

 in which the somites are complete in the young condition, as may be seen in the 

 series figured in Pis. XIIa., XIIb., XIIc, XIId., but that as the animal increases in 

 size the carapace of the cephalon encroaches upon and covers over the surface of the 

 pereion, the dorsal arc of which ceases to be formed ; and thus the carapace appears 

 as part of the pereion which it covers. But this is not always the case, for in the genus 

 Eucopia nearly, if not all, the somites of the pereion are perfect, while the carapace 

 overlies them all. In this case, however, the pereion is of a soft and membranous 

 structure, and has therefore little protective value, whereas in the stronger forms, the cara- 

 pace forming an efficient protection, the inner calcified structure of the somite is not wanted. 



The carapace is also capable of fulfilling offices that simple somites could not carry out. 

 It forms a great shield that is capable of protecting a greater or less portion of the 

 animal, varying from the entire body in some of the Brachyura to but little beyond the 

 cephalon in Lucifer. 



This protective character is further exemplified in the Macrura, particularly in the 

 fast swimming forms, by the development of a long rostrum at the anterior extremity, 

 which is evidently intended to break the force of any body with which it may come 

 into contact, and so protect the eyes and sensory organs from injury. The rostrum may 

 also in some cases be used as a weapon of offence, the teeth that adorn it increasing its 

 value in this respect ; in some cases the latter have a retaining power, when, as in Notho- 

 caris spiniserratus and Odontoh'plius serratus, the teeth are supplemented by numerous 

 small reversed teeth attached to the others. 



The rostrum is generally firmly fixed and rigid, but in one or two genera, such as 

 Pantomus and Rhynchocinetes of A. Milne-Edwards, the rostrum has an arti- 

 culation with the frontal margin of the carapace, and seems to have the power of 

 movement to a slight extent in any direction at the will of the animal ; this modifica- 

 tion can be due only to one purpose, that of receiving the shock of an approaching 

 enemy directly on its point rather than obliquely. 



