286 



THE CRUSTACEA 



reduced proximal segment of the antenna, and the muscles attached 

 to it have been identified with those which move the proximal 

 antennal segment in the lower Decapods. This structure was 

 described by Audouin and Milne-Edwards 

 as a kind of auditory ossicle. 



No trace of the maxillary gland is 

 known to persist in any adult Decapod, 

 though it is frequently well developed in 

 the larval stages. 



Traces of glandular organs, presumed 

 to be homologous with the antennal and 

 maxillary glands, have been observed in 

 embryonic stages in certain other somites 

 of the trunk. In addition, certain other 

 structures are found in adult Decapods, the 

 excretory functions of which have been 

 demonstrated physiologically, although 

 their morphological significance remains 

 obscure. The most important of these 

 are the " branchial glands," which are 

 masses of connective-tissue cells surround- 

 ing the venous channels in the axis of the 

 gills and are devoid of ducts. Other 

 glands of the dermal type also occur in 

 connection with the gills. 



Nervous System. Great differences exist 

 in the number and disposition of the 

 ganglia composing the ventral chain. 

 (A?Sr e Marc hai n ) ing; Among the lower Decapods the six ganglia 

 corresponding to the six abdominal somites 

 are distinct, but those of the cephalothorax may be more or less 

 coalesced. The largest number of distinct ganglia appears to be 

 found in the Astacura, where those of the five posterior 

 thoracic somites are well separated, the remaining cephalotho- 

 racic ganglia being more or less completely coalesced to form 

 a large suboesophageal ganglion. In other cases coalescence 

 has taken place to a greater extent, and in the Scyllaridea and 

 some Caridea, at least, all the cephalothoracic sternal ganglia form 

 a single mass. Among the Anomura the degree of coalescence 

 varies, and sometimes the first abdominal ganglion is approximated 

 to the thoracic mass. Among the Brachyura the concentration of 

 the nervous system reaches its highest point ; the whole of the 

 sternal ganglia are united into a rounded mass lodged in the 

 thorax, from which the nerves radiate outwards. As a rule this 

 mass is perforated in the centre for the passage of the descending 

 artery. T n the more primitive Dromiacea, however, the concentra- 



Fio. 167. 



Diagram of the excretory 

 system of Eupagums bernhardus. 



