I. Morphological and Physiological Works of a General Character. 3 



larvae ; while the position of the notch separating the gastric and rectal portions of 

 the gut is also similar in both. Bury also regards the stalk of the Crinoid larva 

 as representing the pre-oral lobe in the larvae of other Echinoderms ; fixation oc- 

 curring at the anterior end by the pre-oral pit , which corresponds to the large 

 sucker of the larval organ in Asterina gibbosa. Both forms have an unpaired an- 

 terior body-cavity occupying the pre-oral lobe and opening externally by the 

 water-pore ; while the hydrocoel appears at the anterior end of the left peritoneal 

 sac in close connection with the anterior body-cavity, and grows as a ring round 

 the gullet. In both forms again interradially placed basals are developed round 

 the right peritoneal sac in a horseshoe -shaped curve open towards the pre- 

 oral lobe. - - The variable position of the anus in the Echinoderms appears to be 

 due to a secondary migration along the line of the ventral mesentery separating 

 right and left body-cavities. Sometimes it stops in one interradius and sometimes 

 in another, and sometimes it moves on to the aboral surface. 



Semon finds that the bilateral larvae of the Echinozoa have two distinct 

 ciliated bands, a post-oral and an adoral which last sends down a loop into the 

 fore-gut, and in the Asterids it also furnishes the pre-oral ring. In larvae of 

 Echinids and Asterids the nervous system is limited to the ciliated bands , while 

 those of Synaptids and Ophiurids are higher developed with specialised nerve 

 tracts. The primary tentacles of the Holothurians , being the earliest 

 outgrowths from the hydrocoel , are homologous with the water-vessels of other 

 Echinoderms, occupying a similar position with regard to the stone canal. The 

 water- vessels of Echini are in continuation of the primary tentacles , while those 

 of Holothurians alternate with them. Hence the two classes diverged before the 

 development of the water-vessels, when the hydrocoel consisted simply of a ring- 

 canal and 5 primary tentacles. Echinoderms with normal development pass 

 through a bilaterally symmetrical (dipleural) and a bilateral-radial (am phi - 

 pleural) stage. The latter has 5 tentacles covered by a thickened sensory epi- 

 thelium, a ring canal opening externally by a water-pore, and a nerve-ring inside 

 it, while the dorsal mesentery marks a bilateral plane. This Pentactula-larva 

 also occurs in cases of abbreviated development and contains nothing of ceno- 

 genetic origin. All Echinoderms may be derived from such an ancestral form, 

 Pentactea, according to the relative development of the two primary regions of its 

 body, the visceral cup and the tentacles. These regions retain their primitive 

 relations most nearly in Holothurians (Angiochirotae) ; in Echinids the tenta- 

 cular region vanishes and the cup alone remains (Achirotae); while in the brachiate 

 forms (Coelomachirotae), coelom, genitalia, and gut are successively withdrawn 

 into the tentacular region. This becomes proportionately more and more impor- 

 tant and independent , the radiation and decentralisation being most marked in 

 the Asterids, while the Crinoids are the most modified , showing the greatest di- 

 vergence from the ancestral form. The author's general conclusions respecting the 

 ancestral Echinoderms agree very closely with those reached by Walther on palae- 

 ontological grounds [see Bericht for 1886 Ech. p 7]. The fore-, mid- and 

 hind- gut are homologous in all Echinoderms , being taken over by the Pentactula 

 from the bilateral larva. The change from the bilateral to the radial type is uni- 

 versally accompanied by more or less rotation of the axes , due to the change in 

 position of mouth and anus. This seems to indicate that the phylogenetic transi- 

 tion form was attached. The palingenetic nature of the water-vascular 

 system is indicated by the uniformity of its characters in the Pentactula-stage 

 of all classes. The stone-canal, lying along the dorsal mesentery, opens into the 

 water- vascular ring between two primary tentacles , and marks a median plane 

 which is traceable in the adult Echinoderm. These relations, remaining unaltered 



