Class 2. Asteroidea. 131 



are developed only on the ventral (oral) surface, which, unlike that of 

 the Crinoids, is considerably more calcified than the dorsal side. The 

 alimentary canal is very short and almost radially symmetrical. The 

 Asteroidea are divisible into two fairly dissimilar orders, the Star- 

 fish, and the Brittle-stars. 



In some of the Starfisli and Brittle-stars with ^ix arms or m.ore, reproduc- 

 tion by fission has been observed; the disc divides transversely, so that 

 two individuals are formed, each consisting of half a disc and half the number 

 of ai-ms; they attain the perfect fonn later by regeneration. Other Star- 

 fish divide by throwing off the arms; a new individual develops from each, 

 whilst the disc buds out new ones at the old scars. Whilst such an asexual 

 multiplication occui-s in a few forms only, a great capacity for regeneration 

 is common to all, occurring both in Asteroids and Ophiurids ;* lost arms are 

 easily renewed, even if several are destroyed at the same time, and, especially 

 among the Ophiui-ids, there are individuals which ai-e ahnost pei-petuaUy 

 engaged in regenerating lost parts. 



Order 1. Astcrida {Starfish). 



The flattened body consists of a disc with five or more arms, 

 broadest at the base near the point of origin, and narrowing towards 

 the tip. The disc and arms pass directly into each other without 

 any distinct limit. The length of the latter varies very much, for in 

 some they are many times longer than the breadth of the disc, whilst 

 in others they are only just indicated, so that the whole anirhal looks 

 like a pentagonal plate, and there are all possible intermediate 

 forms between these two extremes. 



The mouth which is without armature, lies in the middle of the 

 oral surface ; it leads into a spacious gastric cavity, circular in form, 

 and with much-folded walls, the so-called stomach. This gives oif 

 (sometimes in pairs) tent long-branched caeca, two extending into 

 each arm; they are glandular and pour their secretion into the 

 stomach, A circle of short, and also glandular, caeca arises from the 

 stomach above the large ones, close to the anus; this is a small 

 aperture almost in the centre of the dorsal surface, lying in an inter- 

 radius, close to, but not at, the aboral pole : it is wanting in some 

 forms. The madreporite, which is perforated like a sieve, also 

 lies aborally in one of the inter-radii. Along the ventral side of 

 each arm runs the ambulacral groove, which is continued on 

 to the ventral surface of the oral disc as far as the mouth. The 

 tube-feet are situated in this groove, generally arranged in two, 

 occasionally in four, rows ; each is usually provided with a sucker at 



* Also in the Crinoids. 



f In forms with five arms ; with a larger nvunber of arms there is a corresponding 

 increase in the ntimber of osca. 



K 2 



