ZOOLOGY AND BOTANY, MICROSCOPY, ETC, 651 



notochonl and the dorsal wall of the i)liarynx ; they appear to bo of 

 hypoblastic origin, and it is possible that they are formed at the 

 expense of the notochord. Later on, the notochord increases greatly 

 in size, and becomes vacuolated just like the notochord of young 

 Lampreys and Elasmobrauchs ; the skeletal rods, becoming of con- 

 siderable size, unite anteriorly to form a single bar, while the whole 

 structure forms the support of the proboscis. 



" From its development, position, relations to surrounding parts, 

 histology, and function," Mr. Bateson says, " it aj^pears to me to be 

 comparable with the notochord of the Chordata, and this name is 

 strictly appropriate to it. Even if the suggestions which will be 

 made hereafter as to its phylogenetic significance be not accepted, this 

 rejection would in no way militate against the fact that this structure is 

 to all intents and purposes a notochord, which can only be designated 

 as a longitudinal dorsal sujiporting rod, derived from the hypoblast," 



Particular attention is given to the resemblances between the 

 Enteropueusta and AmpMoxtis ; as to the position and mode of origin 

 of the central nervous system there is great similarity, the only im- 

 portant point of difference being that the invagination of the dorsal 

 cord is only partial in Balanoglossus, but is complete in Amjohioxus. 



Echinodermata. 



Phylogeny of Echinoderms.* — Mr. H. F, Nachtrieb records some 

 observations made on the development of many of the Echinodermata 

 of Beaufort, and concludes with some remarks on the phylogeny of 

 Echinoderms. If we compare the origin of the body-cavity and water- 

 vascular system in the different classes, we see that in the Holo- 

 thurians we have one median pouch given off from the enteron, 

 and that it, by division, gives rise to the body-cavity and water- 

 system. In the Echinoids there is a two-horned pouch given off. In 

 the star-fish there are two separate lateral ijouches given off, of which 

 the left gives rise anteriorly to the water-system, and the right and 

 the posterior part of the left become the body-cavity. In Ophiurids 

 so far as known, there are two separate pouches, both of which divide, 

 the anterior part of the left becoming the water-system, the anterior 

 of the right atrophying, and the posterior parts of the right and left 

 becoming the body-cavity. In the Crinoids there are first given off 

 two separate ponchos, which become the body-cavity, and then a 

 single one, that becomes the water-system. Assuming that the story 

 of the Ophiurids and Crinoids is correct, we have here a rising scale, 

 in Avhich the Holothurians occupy the lowest, the star-fish the middle, 

 and the Crinoids the highest position. In favour of this there are 

 some anatomical facts. 



The objections of palajontology are not very difficult to answer. In 

 assuming the Holothurians as the primitive forms it is not necessarily 

 implied that the line of development is a straight one, as it is rcj^re- 

 sentod above. It is quite probable that the line began to break with 

 the appearance of the star-lish. 



* JoliriB-IlopkirjH Univ. Circ, iv. (1885) pp. G7-8. 



