194 THE VOYAGE OF H.M.S. CHALLENGER. 



brandies of the water-vessels are therefore either absent altogether as in the Blastoids 

 and in some of the arms in many Actinometrce ; or they supply delicate papillate tubules, 

 the tentacles, the chief functions of which are probably those of respiration and sensation. 

 Ludwig's researches have demonstrated that the primary water-pore of the Antedon-lavva 

 is homologous with the commencing madreporite of an Echinozoon. But in no Starfish or 

 Urchin is the communication between the water- vascular system and the exterior effected 

 through the intervention of the body-cavity as it is in the adult forms of recent Crinoids, 

 whatever be the condition of the larva. There is good reason to believe that the 

 hydrospires of the Blastoids and Cystids were neither connected with the water- 

 vascular ring nor with the body-cavity, so that the absence of a continuous madreporic 

 canal may be regarded as eminently characteristic of the Pelmatozoa, though it occurs in 

 some Holothurians and jiossibly also in certain Ophiurids. 



The limitation of the functions of the water-vascular system in the Pelmatozoa, 

 although a natural consequence of the presence of the stem, is in no way connected with 

 this organ morphologically. With the blood-vascular and nervous systems, however, the 

 case is different. 



There can be little doubt that the remarkable neuro-vascular axis which occupies 

 the central canal of the stem of a Neocrinoid, occupied a similar position in the Palseo- 

 crinoids. Even in the case of sessile forms like Edriocrinns there must have been a 

 chambered organ, from the fibrillar envelope of which were derived the axial cords of the 

 rays and arms, just as in the recent Holopus. The Blastoids aU have basal and radial 

 plates, and, with the exception of the Astrocrinidse, a perforated stem, so that one can 

 hardly be going too far in assuming that its central canal lodged a neuro-vascular axis, 

 as in recent Crinoids, The same is probably true of the pedunculate Cystids ; though I 

 much doubt whether the Agelacrinidee had a chambered organ. This peculiar structure, 

 which is so important a part both of the blood-vascular and of the nervous systems, is 

 essentially anti-ambulacral, being developed in the right larval antimer. With its 

 connections it is as important a part of the organisation of a Pelmatozoon as the cerebro- 

 spinal axis is in a vertebrate animal ; and there is in many respects a striking analogy 

 between the two. 



Although in such intimate relation with the basal and radial plates, the chambered 

 organ cannot be correlated -ftdth their presence ; for true homologues of these plates occur 

 in the Echinozoa, in none of which have any traces of an aboral neuro-vascular centre yet 

 been discovered. 



There are two characters, however, by which some or all of the Pelmatozoa are 

 especially distinguished, viz., (1) the presence of a stem, and (2) the development of arms 

 upon the primary radials, with muscular articulations between their component joints. The 

 majority of the Pelmatozoa have both stem and arms, but the Astrocrinidae seem to 

 have had neither, though possibly stalked when young ; while the remaining Blastoids 



