544 COMPARATIVE ANATOMY chap. 



trabecule within the system of canals increase in number. In the tentacles, the 

 following changes have taken place. Formerly the twenty-five tentacles were 

 arranged in five radial groups of five each. The five tentacle canals of a group were 

 connected by a common tentacle canal rising with the circular canal. Now all the 

 five tentacle canals of a group rise separately from the water vascular ring. 



Further, during the period of the attachment of the stalked larva, four new stone 

 canals appear, and four new calyx pores form in the other interradii. These and 

 all that arise later cannot, of course, form in the same way as the primary calyx 

 pore. 



The stage, the development of which has just been described, has been called 

 the Cystid stage, owing to the absence of arms, to there being no clear division 

 of the calyx into dorsal cup and tegmen calycis, and to the occurrence of the 

 rudiment of the genital organs as an axial organ, whereas later the genital glands lie 

 in the arms and especially in the pinnules. 



In opposition to the above view it may be remarked : 



1. That neither the want of arms nor the absence of division of the calyx into a 

 dorsal cup and an ambulacral disc is characteristic of the Gystidea. 



2. That the skeletal system of the attached larva of Comatula is altogether 

 radiate, consisting of the three circles, the radials, the basals, and the infrabasals. 

 On the other hand, the irregular arrangement of the skeletal plates is, as a rule, 

 characteristic of the C'l/slidea. Those Cystids which most resemble the larva of 

 Comatula in the number and radial arrangement of the skeletal plates are also those 

 which are, of all Cystids, the most nearly related to the Criuoidea. 



3. The hydroccel of the stalked Comatula larva consists simply of the water 

 vascular ring and a circle of tentacles, which receive their canal direct from the water 

 vascular ring. In the Cystidea, radial canals must have run out from the water 

 vascular ring, below the food grooves of the ambulacra, giving off tentacle canals to 

 right and left, and also probably penetrating into the arms. 



4. The appearance of the first rudiments of the genital glands in the body merely 

 proves that the definitive position in the arms is secondary, and this applies to all 

 Echinoderms. 



The position of the anus, indeed, agrees in both. 



5. Last Stage of the Attached Stalked Larva — Pentaorinus Stage. 



[Cf. Fig. 326, p. 375.) 



This stage is distinguished by the rise of the arms, which begin to grow out in 

 the radii between the circle of the orals and that of the basals. Each rudiment of 

 an arm is, from the very first, supported on its apical side by a newly arising skeletal 

 plate. These plates are the five radials of the dorsal cup. Distally from each 

 radial on the growing arm, two new skeletal plates follow one another, the first and 

 second costals. The growing rudiment of the arm then forks, the distichals form, 

 and so on. 



During the formation of the arms the five middle and strictly radially arranged 

 tentacle canals of the five tentacle groups become the radial vessels, which fork with 

 the arms. Fresh investigation of this point is, however, much needed. 



The interval between the oral pyramid and the bases of the arms increases, and 

 the tegmen calycis thus comes into existence. The pyramid of five oral valves in 

 the middle of the latter does not grow further, and the valves with their skeletal 

 plates finally disappear. Eound the anus, which comes to lie in the tegmen calycis, 

 an anal plate develops temporarily. 



At this stage the resemblance of the attached and stalked larva of Antedon 



