548 COMPARATIVE ANATOMY CHAP. 



ventral wall of the left anterior crclom in front of it, embedded itself to a certain 

 extent from without in the cwlomic vesicle, so that this vesicle surrounded it in the 

 shape of a horse-shoe. Round the mouth, the body wall (and with it the left anterior, 

 ccelomic vesicle which lay here) grew out into five tentacles which, as in so many 

 attached animals, served for bringing in food, for the sense of touch, and for respira- 

 tion. (Compare the tentacles and the horso-shoe-shaped tentacle - carrier of the 

 Bryozoa Cephalodiscus, etc.) Thus, the left anterior ca'loru, which from the very 

 first was, like the right, connected with the exterior by means of a canal, produced 

 the primary horse-shoe-shaped hydrocoel with the primary tentacles and the hydro- 

 pore (stone canal). In this way the first impulse towards the development of the 

 radiate structure was given. The horse-shoe finally closed to form the circular canal. 



The right anterior side of the body, which was used for attachment, could be 

 produced into a stalk, as is the case in most Pdmatozoa. (The larval organ of 

 Asterina may be a modified reminiscence of such a stalk.) The right anterior 

 ccelomic vesicle, which lay in this region, now serving for attachment, lost its efferent 

 aperture, atrophied, or became a cavity of the stalk (chambered sinus and its con- 

 tinuation in the Crinoids(\), c<-lom of the larval organ in Asterina (?). 



The body now developed principally in the oral ami tentacular region (on the left 

 anterior side). Tlie posterior portion of the body with the anus near its end was 

 originally like a lateral outgrowth or shield on the body, which gradually subsided 

 and disturbed less and less the radiate appearam -c. 



According to this view, the greater development of the anal interradius which is 

 found in many Crinoids, especially in paheozoic forms, may possibly be an original 

 condition, in connection with which we have the occurrence of special anal plates in 

 the anal interradius. The anus also may originally have lain outside of the circle of 

 tentacles, a supposition which harmonises with its position in the Cystitis and in 

 the ontogeny of Antedon. 



Concurrently with these changes, the left posterior c-<i-lom, which lay nearer than 

 the right to the mouth, which had shifted to the left, now upper, side, grew round 

 the oesophagus, and forming a vertical mesentery, became the oral en-lorn. The 

 right ca-lomic vesicle, however, spread out chiefly in the lower (originally the right) 

 region of the body, and became (also forming a vertical mesentery) the apical crelom. 

 The mesentery dividing the two (oral and apical) sections of the crelom would natur- 

 ally be horizontal (transverse). 



In the vertical mesentery, the rudiments of the gonad (the axial organ) arose 

 as a ridge-like thickening and growth of the endothelium on one side ; in mature 

 animals, this opened outward through a genital duct and aperture in the region be- 

 tween the mouth and the anus. 



This phyletic stage, deduced as a result of the attached manner of life, may be 

 called the Pentactsea. 



For the protection of the body, calcareous plates developed in the mesenchyme 

 below the integument, at first, perhaps quite irregularly. 



From the Hypothetical (unknown) Pentactaea Stage to the known 



Echinoderm. 



Most Echinoderms gave up the attached manner of life at a later stage. The 

 known case of Antedon, in which an animal in the highest degree adapted for the 

 attached life resumed the free life, is specially welcome and useful in this connec- 

 tion. 



The ancestors of the Holothurioidea were probably the first to renounce the 

 attached manner of life, although not as early as the Pentactaea stage. 



