436 CRUSTACEA MALACOSTRACA. 



Some Malacostraca (e.g. Tanaidacea, Cumacea) are small, 

 but many attain a much larger size than any of the Ento- 

 mostraca. 



In contrast with the varying number of segments in the post- 

 cephalic region of the body met with in the several groups of 

 the Entomostraca, and especially in the Phyllopoda, the Mala- 

 costraca possess a constant number. Eight segments are 

 found with great uniformity in the thorax, and seven, in- 

 cluding the telson, in the abdomen. These regions are clearly 

 marked by the character of their appendages, and frequently 

 by the difference in the mobility of their segments, those of 

 the abdomen being the most mobile. The only exceptions 

 to uniformity in the number of segments are met with in the 

 Leptostraca (Nebalia and its allies) which- are in many respects 

 intermediate between the Phyllopods and the Eumalacostraca, 

 and have 8 segments in the abdomen ; in some aberrant forms 

 of Amphipoda, whose relation to the main body of the order 

 which conform to rule is undisputed ; and in the Decapod Leucifer, 

 in which the eighth thoracic segment is not differentiated, and 

 its appendages and those of the seventh segment are absent. 



In the stereotyped number of the segments of the regions of the body 

 the Malacostraca may be compared with the Insecta, which occupy a 

 corresponding position at the head of the Antennata. A similar uniformity 

 in the number of segments, which presents great variation in the lower 

 members of a phylum, is found in the cervical region of mammals 

 among Vertebrates. 



The head is marked off from the thorax by the character of 

 its appendages, and in the least differentiated Malacostraca 

 Nebalia, (?) Anaspides and the Holotrophous Schizopods * 

 by a groove between it and the first thoracic segment. 



A dorsal shield is present in many groups of Malacostraca 

 investing some or all of the segments of the thorax. In Nebalia 

 and the Lophogastridae the shield appears to be a purely cephalic 

 structure, a fold of the integument of the dorsal and lateral 

 regions of the head, and the thoracic segments, though covered 

 by it, do not participate in its formation. In the other shield- 



* In Sars' figure of Gnathophausia longispina, in the Challenger 

 Monograph on the Schizopoda (PI. 8, Fig. 17) the first thoracic segment 

 appears to be limited in front by a definite groove, which would thus 

 separate the cephalic and thoracic regions. If this is the case the dorsal 

 shield in this genus is a purely cephalic structure. 



