228 SUMMARY OF CURRENT RESEARCHES RELATING TO 



the circle of tentacles. Yolky detritus is still present in the coelenteron. 

 On the fourth day the base of this invagination degenerates, and so the 

 coelenteron communicates with the exterior. 



While the mouth invagination is still in process of formation, the 

 endodermic portion of the mesenteric filaments arises on the six ventral 

 mesenteries by a proliferation of cells on the upper part of the free 

 edge. On the fifth day the ventral mesenteric filaments are completed 

 by strap-like ectodermic down-growths from the stomodseum over the 

 endodermic thickening on each mesentery. On the sixth day the dorsal 

 mesenteric filaments arise. The dorsal and ventral mesenteric filaments 

 appear homogeneous in origin, though of diverse function. On the 

 seventh day the eight filaments reach about half-way down the free 

 edges of their respective mesenteries. 



Further development consists of elaboration of the organs already 

 present. At the end of the third week the first bud is formed, and the 

 soUtary polyp becomes a young colony by stolonial gemmation. The 

 young colonies were successfully fed in the laboratory on larvas and 

 adult individuals from colonies of LeptocUnum and Botryllus. 



Monograph on Pennatulacea.* — Sydney J. Hickson has completed 

 a magnificent monograph, dealing with the Pennatulacea of the Siboga 

 Expedition — the richest collection of sea-pens that has ever been made 

 by a single scientific expedition. Seventeen genera and forty-five 

 species are represented. Many anatomical and histological descriptions 

 are given, and attention may be directed to the investigation of the 

 structure and distribution of the radial canals in Virgularia, Anthoptilum, 

 and Osteocella ; of the structure of the " mesozooids " of Pennatula 

 murrayi ; and of the structure and relations of the remarkable brown 

 tubes of the same species. Mesozooids have been found only in a few 

 Pennatulids ; they differ from autozooids and siphonozooids, and appear 

 to be exhalaut in function. They have no tentacles, mesenteric 

 filaments, or gonads. 



The " brown tubes " form a series of bent tubes lined by a specialized 

 epithelium, forming a means of communication between the sub- 

 cutaneous system of endodermal canals (the " solenia ") and the lateral 

 longitudinal canals. In unstained sections they are brown. The 

 epithelium lining the tubes is columnar and undoubtedly ciliated. At 

 each end of the tube there is what appears to be a tumid rosette-shaped 

 funnel mouth on which the cilia are usually well preserved. These 

 funnels recall nephrostomes. It may be that the brown tubes bring the 

 mesozooids into relation with the dorsal longitudinal canal. 



The gonads of many species were examined, and the general results 

 arrived at are (1) that all the species are dioecious, (2) that all the 

 species are oviparous, and (o) that the sexual organs may reach maturity 

 long before the full size of the colony is attained. No evidence was 

 found of protandry or protogyny, nor a single case of an egg that has 

 begun to segment within the body-cavity of the parent. There is an 

 interestius: discussion of the evolution of the Pennatulacea. 



* 



Siboga-Expeditie, Monograph xiv. Leiden : 1916, x and 255 pp. (1 chart 

 10 pis., and 45 figs.). 



