64 SUMMARY or OUKRENT RESEARCHES RELATING TO 



or, in other words, at a time when the hydrocoel consisted only of a 

 circular canal and five primary tentacles. All difficulties are evaded if 

 we suppose that divergence arose from an earlier and simpler stage of 

 development, and one which is retained in the young Echinus and, with 

 slight modifications, in the young Synapta ; this will be again found in 

 the ontogeny of other classes of Echinoderms. The primitive form 

 may be called the Pentactula. 



This phase of development is characterized by the fact that the 

 dipleural larva has begun to confuse bilateral with radial symmetry by 

 the development of the five primary tentacles. At first the radial 

 symmetry afi'ects only one system of organs — the water-vascular 

 — but the nervous system is soon likewise afiected; the bilaterally 

 symmetrical larva may be called the Bipleurula. It may be said that 

 all Echinoderms, save where their development has been cenogenetically 

 shortened, pass through two larval stages, one bilaterally symmetrical 

 and one bilateral and radial. It is especially during the latter that the 

 internal and external resemblance between the larvae of different classes 

 is considerable. 



The Pentactula may be regarded as a creature whose anterior pole is 

 marked by the mouth-opening. Around the mouth are five tentacles, 

 formed as outgrowths of the water-vascular ring which surrounds the 

 pharynx. Over these outgrowths the outer skin forms a thickened 

 sensory epithelium. From the ring a canal leads to the surface of the 

 body, and this canal, the primary stone-canal, opens by the dorsal pore 

 freely to the exterior ; as this pore is always found on the dorsal side in 

 the bilateral early stages of Echinoderm-larvge, it is called the dorsal 

 pore. In front of the water-vascular ring there is a nervous ring which 

 surrounds the pharynx ; it gives off five nerves to the primary tentacles, 

 on the inner side of which the nerves lie. The nerves as well as the 

 nerve-ring, whose derivates they are, lie superficially in the ectoderm, 

 from which they are derived. The enteric canal consists of oesophagus, 

 mid-gut, and hind-gut ; the anus lies on the ventral side, and may 

 approximate to or remove itself aw^ay from the mouth, so that, in 

 extreme cases, it comes to lie within the circlet of primary tentacles or 

 at the hinder end of the body. Between the gut and the body-wall 

 there is a wide body-cavity, formed from symmetrical enteric sacs ; there 

 is a dorsal mesentery which gives a distinct sign of the bilaterally 

 symmetrical origin of the coelom. The primary stone-canal arises from 

 the circular canal between the points of insertion of two primary 

 tentacles ; this character gives a plane of symmetry for the Pentactula, 

 and passes through the dorsal mesentery, dividing the gut in the median 

 line and that tentacle which may be called the ventral tentacle. 



This larval form exhibits no characters which can be regarded as 

 cenogenetic, and if we suppose that the stem-form of the Echinodermata 

 was a creature which, in external form and internal organization, had 

 great resemblance to it, we may derive all the classes of the Echino- 

 dermata from a form which may be called Pentactsea. Dr. Semon 

 thinks that a derivation of this kind agrees with the facts of comparative 

 Anatomy, and offers the key to some unsolved problems. 



When we come to consider the divergencies which obtain among the 

 various classes, we see that one group — the Holothurians — have retained 

 essentially the relation of the body to the primary tentacles which we 

 saw in the stem-form ; as this tentacular system has remained as essentially 



