No. 19.] ECHINODERMS OF CONNECTICUT. 95 



The reproductive organs are likewise modified, for the pres- 

 ence of the anus in the posterior interradius prevents the develop- 

 ment of the gonad which in the regular urchins occupies this 

 position. There are thus only two pairs of gonads developed in 

 the adult with four symmetrical openings on the plates at the 

 apical pole of the test, as shown in Fig. 14. 



In the key-hole urchin the mouth lies near the center of the 

 oral surface, as in the sea-urchin and sand-dollar, but the position 

 of the anus is different from that which is found in any other 

 echinoderm of the region, being likewise situated on the oral 

 surface, and only a short distance posterior to the mouth (Plate 

 XXIV, fig. I). 



The mouth is provided with five massive jaws, each 

 ending in a sharply pointed tooth. The esophagus (Plate 

 XXIV, fig. i) leads towards the aboral surface, and then bends 

 sharply anteriorly until it reaches the circle of supporting plates 

 which surround the jaws. It then divides, as in Arbacia, into 

 two branches, one of which forms the broad, flat stomach, while 

 the other, which is closely attached to the inner margin of the 

 stomach, is known as the siphon. The stomach (st) is just 

 broad enough to occupy all the space between the inner circle 

 of supporting plates and the outer system of plates which pass 

 between the oral and aboral surfaces of the body. 



The outer margin of the stomach is, as in Arbacia, provided 

 with numerous sac-like lobes which greatly increase the secretory 

 surface. 



Explanation of Plate XXIV. Anatomy of Disk-urchins, Mellita and 



Echinarachnius. 



Fig. I. Mellita pentapora. Dissection of body to show alimentory canal. 

 The slender esophagus leads from the centrally placed 

 mouth (surrounded by five large jaws) to the broad stomach 

 {st) and siphon (siph). The convoluted intestine (int), fol- 

 lows the definite course indicated, and passes gradually into the 

 rectum (r), which opens through the anus (a) situated between 

 the mouth and the median lunule. In some specimens the anus 

 is placed at the inner end of the lunule. The five lunules (/) 

 are shown. 



Fig. 2. Echinarachnius parma. Dissection of the body to show alimentary 

 canal; e, esophagus; st, stomach; siph, siphon; int, intestine; 

 r, retum, opening on oral surface near margin of disk; s. c, 

 madreporic canal. The five angular jaws are situated sym- 

 metrically about the mouth. 



