THE MYSIDACEA 



179 



no sensory setae could be discovered, it seems possible that this 

 pit may represent the antennular statocyst of Syncarida and 

 Decapoda. 



Reproductive System. — The two tubular ovaries are connected 

 with each other by a narroAv l^ridge. The oviducts proceed from 

 their hinder ends and probably open in the usual position at the 

 base of the sixth thoracic limbs. The testes are closely approximated 

 in the middle line and consist each of a number of pyriform follicles 

 opening into the vas deferens, which is dilated near the front of 

 each testis to form a seminal vesicle. The external openings are 

 situated on papilliform elevations at the bases of the last pair of 

 thoracic limbs. The spermatozoa have the form of slender rods 

 each with a filiform tail attached at an acute angle at one end. 



-t. 



Fig. lOS. 



A, telson and one uropod of Mysis, from above. The marginal setae of tlie uropod are 

 omitted. B, the .statocyst, seen in optical section from the side, further enlarged, en, endo- 

 jjoilite of uropod, containing the statocyst near its base ; cr, exopodite of uropod ; n, nerve 

 supplying sensory setae of statocyst ; st, statolith ; t, telson. (After Sars.) 



Development. — The whole course of development takes place 

 Avithin the brood-pouch. In the Mysidae segmentation is of the 

 discoidal type. The embryo becomes freed from the egg-membrane 

 after the appearance of the first three pairs of appendages, at which 

 stage, corresponding to the nauplius, the first larval cuticle is 

 formed (Fig. 109). The caudal region, which within the egg was 

 flexed ventrally, becomes extended and the body acquires a slight 

 dorsal curvature. An important feature is the presence of two 

 immovable setose styles (/) terminating the abdomen and re- 

 presenting the caudal furca of the Leptostraca. Under the cuticle 

 of this maggot-like nauplius stage the remaining cephalic and 

 thoracic appendages (th) appear simultaneously, followed by the 

 uropods and, after an interval, by the pleopods. The elongation of 

 the body in the post-naupliar region is effected by successive divisions 

 of a series of teloblastic cells as in Isopoda. A pair of lateral 



