916 EEPORT— 1898. 



as the notochord is eventually replaced by mesoblastic supporting tissue, so the 

 chordoid walls of the pharyngeal cleft give place to mesoblastic branchial arches. 



The Echinoderinata are a group which have long been under suspicion as con- 

 nected with the genealogy of the Vertebrata. A large proportion of them have 

 free-swimming pelagic larvae, which feed by ciliary ingestion. If the processes of 

 ingestion be followed by feeding early Bipinnaria in the laboratory, the food 

 particles may be seen to pass down the ventral part of the so-called cesophagus and 

 to accumulate at its inner end. They are then periodically injected through the 

 small aperture into the ' stomach.' The ventral groove has already been compared 

 by Mr. Garstang to the endostyle of Tornaria and to that of the other Chordata. 



On the other hand, the water current, after passing down the gullet, returns 

 along the dorsal part and is discharged to the exterior by a pair of lateral grooves 

 which become Very marked in later larvae. This dorsal part is kept open by the 

 arched roof of the gullet, the cells of which exhibit a histological structure closely 

 approximating to that of the typical chordoid tissue. Bearing in mind the other 

 features in which the Echinodermata are acknowledged to resemble the Chordata, 

 there seems ground for homologising this dorsal chordoid element of the larval 

 gullet with the notochord of the latter group, and the two lateral grooves with the 

 pharyngeal clefts. They apparently represent an even more primitive stage of these 

 organs than is found in Cephalodiscus, in which the chordoid organ has apparently 

 diverged into two and the atrial grooves bave become definite clefts. 



Briefly, then, the phyletic history of the vertebrate notochord and gill slits may 

 be summed up as follows. In the pelagic ancestor of the Chordata the gut was 

 undifferentiated, and the food and water were alike washed through its course. 

 An early constriction between pharynx and stomach resulted in the exclusion of 

 the water current from access to the latter, and the consequent return of the same 

 along the dorsal part of the former. In relation to this the cilia became confined 

 to the ventral part, eventually giving rise to the endostyle, whilst the dorsal part, 

 supplied with ' atrial ' water alone, became modified into the notochord. Under 

 conditions of insufficient nutrition the constituent cells of this area lost their cilia, 

 and, undergoing a retrogressive metamorphosis, they became a mass of vacuoles and 

 cells with little, if any, residual protoplasm ; between the two portions of the gullet 

 so formed there appeared the lateral ' atrial ' grooves. Such a condition is exhibited 

 in echinoderm larvae. 



In such a form, as Cephalodiscus, where the original functions persist, the noto- 

 chord, retaining its connection with the ' atrial ' apertures, divides into two, but in 

 the direct line of chordate descent it remains single. 



Further diflferentiation leads eventually to the separation of the notochord 

 dorsally, and that of the endcstyle (or thyroid) ventrally, from the pharynx, whilst, 

 at the same time, the atrial grooves become transformed into pharyngeal clefts (nr 

 gill slits). 



Such a hypothesis, in attempting to account for the origin of notochord and of 

 gill slits, agrees fairly with the facts of morphology and ontogeny, although it can 

 in no way be regarded as entirely conclusive. 



3. Le Developpement da Coeur chez les 2'umciers : Quelques Considera- 

 tions sur la Phylogenic des Ascidies simples. Par Professeur Cii. Julin. 



4. Demonstration of Dr. Field's Card-catalogue of Zoological Literature. 



By W. E. HoYLE. 



5. A Phylogenetic Classification of the Pelmatozoa. 

 By F. A. Bather, ALA., F.G.S. 



The classification is an epitome of that adopted in a forthcoming text-book of 

 zoology edited by Professor Ray Lankester. The standpoint is phylogenetic, and 

 is based on the following beliefs. 



