VIII ECHINODERMATA—PHYLOGENY 549 



The organisation of the Pentacta'a would only become conii)letely adapted to the 

 attached manner of life, when the number of tentacles and the surface for taking in 

 nourishment increased. Such increase might take place in various ways ; we have 

 many examples among attached animals belonging to other divisions of the animal 

 kingdom. 



The PentactiKa may have become perfected in one direction as follows : — 



The interval between the bases of the primary tentacles and the mouth increased, 

 while the (apical) interval between the primary tentacles and the attached pole 

 remained the same or decreased. By the shifting of the circle of tentacles away 

 from the mouth, the basal piece of each tentacle canal would be drawn out radially 

 below the oral body wall, and would become a radial canal, from which new lateral 

 tentacle canals would bud out alternately to right and left, alwaj's proximally to 

 the shifting j^rimary (now terminal) tentacle, the body wall projecting in the form 

 of tentacles. There thus arose in each radius a double row of tentacles, which, to 

 speak exactly, stood in the corners of a zigzag line. Nutritive particles, descending, 

 would be sent on to the mouth between these rows of tentacles. This adaptation 

 became perfected when the floors between these rows of tentacles sank in the form of 

 furrows — the food grooves or ambulacral furrows ; these furrows then became pro- 

 vided with means of transport, in tliis case with cilia. The epithelial cells lining 

 the furrows gradually became sensory cells, and epithelial nerve ridges, — the radial 

 nerves, — would arise, which would meet round the mouth as the nerve ring. 



This rise of the radial nervous system of the Echinodermata as a natural 

 development of the ambulacral furrows, and of the palisades of sensory tentacles border- 

 ing these furrows on each side, may be without difficulty followed out in detail, 

 although there is no space for such an attempt in this volume. 



At this stage are found the armless Cystids with their carapace of plates. 

 The genital organ is enclosed in the body, and opens outward through a single 

 aperture (the " third " aperture of the Cijstidea). 



It was, perhaps, at a similar i)hyletic stage that the ancestors of the Holotlmri- 

 oidca gave up the attached manner of life. For locomotion, they used the tentacles, 

 arranged in five meridional double rows, the body elongated, and the anus shifted 

 to the apical end, which became free. Food was taken in directly at the mouth ; to 

 assist in alimentation, the cesophagus became modified into a pharynx, and the ten- 

 tacles lying close to the mouth became specialised. The food grooves lost their 

 function, leaving, however, behind them the epithelial nerve ridges, which continued 

 to innervate the tube-feet. The food grooves could now close over to form a tube 

 for the protection of the nerve ridges ; these latter became the subepithelial radial 

 nerves, and the lumina of the closed tubes the epineural canals. According to this 

 view, therefore (and this applies also to the Ophiuroidea and Echinoidm), the sub- 

 epithelial radial nerves, with their epineural canals, are the original food grooves 

 closed over to form tubes. They gave up their original function as " food grooves " 

 in proportion as, with the adoption of a free manner of life, food was taken in direct 

 at the mouth. 



It is a question of subsidiary importance, in the derivation of the Holothurid 

 body, which still possesses the single genital, organ and the single aperture, whether 

 the condition of its skeleton is to be regarded as original, or whether it has not rather 

 been derived from the carapace of plates of a Cystid-like animal by means of the 

 multiplication of the skeletal pieces, their loose arrangement, and their decrease 

 in size. 



The longitudinal and circular musculature may be new, but may just as well have 

 been inherited from an attached ancestral stage, in which they could be functional, 

 just as are the longitudinal and circular muscles of Actinia. 



After the foregoing description it is obvious that the Paractinopoda [Synaptidce) 



