412 CRUSTACEA ENTOMOSTRACA. 



Powerful organs of attachment are placed on each side of 

 and behind the mouth ; they consist of two parts (1) of an 

 anterior pair of appendages which apparently correspond to the 

 second maxillae and are in Argulus modified into large sucking 

 discs ; the hook-bearing terminal portion which is present in the 

 newly hatched larva, and in Dolops throughout life, being 

 reduced ; and (2) of a posterior pair, which corresponds to the 

 maxillipeds and is provided with numerous spines on its broad 

 basal portion and a tactile protuberance and two curved terminal 

 claws at its extremity. Xext to these come the four paired 

 swimming feet of the thoracic region, which, with the exception 

 of the last, are, as a rule, covered by the sides of the cephalo- 

 thoracic shield. Each of these consists of a large basal portion, 

 consisting in the adult of several segments, and two much 

 narrower rami, which are beset with long swimming setae 

 and in their form and setigerous investment are not unlike 

 the biramous appendages of the Cirripedia, being like them 

 derived from the copepod feet of the larva (Fig. 267). 



The internal organization recalls that of the Phyllopoda. 

 The nervous system is distinguished by the great size of the 

 cerebral ganglion, and by the close approximation of the six 

 ganglia of the ventral chain. Two large compound lateral eyes 

 are present in addition to the unpaired tri-lobed median 

 eye. The alimentary canal consists of a short arched ascending 

 oesophagus, a wide stomach with two lateral ramified appendages, 

 and a rectum which runs directly backwards and opens to the 

 exterior in the median indentation of the caudal fin above the 

 two divisions of the caudal fork. The shell gland has a well- 

 developed end-sac and opens, not as in most Entomostraca 

 (but cf. footnote, p. 411) on the sucking disc (rax"), but at the 

 base of the maxilliped. The heart is a tube suddenly expanded 

 behind, extending from the region of the brain to the base of 

 the caudal fin, against which it ends abruptly. The blood 

 circulates in sinuses. It leaves the heart by anterior and 

 posterior apertures and returns to it by a ventral median and 

 two posterior lateral ostia (Leydig). The entire surface of the 

 cephalothorax functions as a respiratory organ. There seems, 

 however, always to be a strong current of blood in the caudal 

 fin, so that this part of the body may be regarded as especially 

 respiratory a circumstance from which the group is named. 



