I. MOEPHOLOGT OF NePHEOPS ANDAMANICA. 



1. Of the general construction and segmentation of the Body. 



In a typical Decapod Crustacean, such as Nephrops andamanica, the body 

 is elongate and is manifestly divided into two regions, an anterior region or 

 cephalothorax and a posterior region or abdomen, of which the latter is the longer. 



The cephalothorax is covered, as to its back and sides, by a shield or 

 carapace, so that its segmentation — or subdivision into somites — is not immediately 

 apparent ; but the abdomen obviously consists of a series of seven segments, and 

 therefore bespeaks our attention first. 



Any one of the seven freely-movable abdominal somites, except the last, is 

 seen to be a ring, with a pair of appendages attached to its under surface. The 

 dorsal arc of the ring is wide fore-and-aft and is known as the tergmn, the 

 ventral arc is narrow fore-and-aft and is known as the sternum, while the over- 

 lapping sides of the ring are called the plexira : furthermore, the narrow corner 

 between the articulation of the appendage and the pleurum of either side is 

 termed the epimerum. 



On turning to the cephalothorax, the most that we can distinguish, before 

 the removal of the carapace, are the narrow sterna of some of the thoracic 

 somites ; and even when we have removed the carapace, and have cleared away 

 the gills that lie beneath it, we shall at first sight find, instead of the succession 

 of rings that the abdomen shows, merely the greatly-extended epimera of some 

 of the thoracic somites. 



The cephalothorax is, however, as an examination of its sterna and append- 

 ages reveals, made up of 14 segments, all of which are immovably fused together, 

 many of which are contracted and foreshortened and obscured, and none of which 

 — with the exceptions presently to be noticed — possess identifiable terga like those 

 of the abdomen. The place of terga and pleura is taken by the carapace, the 

 main dorsal mass of which is generally supposed — though it is not certain that 

 this explanation covers all the facts — to be an enormous extension backwards of 

 those elements of the 3rd (antennal) and 4th (mandibular) somites of the body. 



The carapace, it is to be observed, does not merely serve as a cover to the 

 united segments of the cephalothorax, but it overlaps those segments so as to 

 enclose on either side of them a space in which the gill-plumes he, this space 

 being known as the branchial chamber. Moreovei-, it is produced forwards in the 

 middle line, to form a stout bayonet-like organ of defence — the rostrum. 



Although the cephalothorax is one composite whole, it consists of two ill- 

 defined regions, an anterior cephalic region or head and a posterior region or 

 thorax. The boundary line between the two regions is marked on the carapace 

 by a deep semicircular groove — the cervical groove ; and when the carapace is 

 removed, the division between the two regions is still obscurely indicated by a 



