TIIOEACOSTEACA. 465 



The ducts of the very numerous, multilobed hepatic caeca open into 

 the anterior part of the elongated intestine. 



A simple or looped glandular tube (the green gland} opens on the 

 basal joint of the posterior antenna. A shell gland is not developed. 

 / The nervous system is distinguished by the size of the brain, 

 fwhich is placed far forwards and gives off nerves to the eyes and 

 [antennae. The ventral cord, which is connected with the supra- 

 i O3sophageal ganglion (brain) by very long commissures, presents very 

 different degrees of concentration. In the brachyurous Decapods 

 this concentration reaches its highest point, all the ganglia being 

 fused together to form one great thoracic ganglionic mass. The 

 system of visceral nerves is also very highly developed. 



Sense organs. The eyes are large and facetted. Except in the 



Ov 



FIG. 365. Generative organs of Astacug. a, Female ; b, male. Ov, ovaries; Od t oviduct; 

 Va, vulva on the basal joint of the third pair of ambulatory legs (-F'"); 2 1 , testis; Vd, 

 vasdeferens; Oe, genital openings on the basal joint of the fifth pair of ambulatory 

 legs I?*). 



Cumacea, in which the eyes are sessile, they are borne on movable 

 stalks, which morphologically are to be regarded as the lateral parts 

 of the anterior region of the head which have been segmented off. 

 In the larva a median simple eye, equivalent to the unpaired Ento- 

 mostracan eye, may appear between the stalked facetted eyes. In 

 exceptional cases the adult animal may have paired eyes at the sides 

 of the thoracic appendages, and unpaired eyes between the abdominal 

 feet (Euphausia). Auditory organs are wanting in the Cumacea 

 and Stomatopoda. In the Decapoda they are present as vesicles 

 containing otoliths in the basal joint of the anterior antenna, and in 

 many Schizopoda in the lamellae of the caudal fin. The delicate 



30 



