438 SUMMARY OF CURRENT RESEARCHES RELATING TO 



is in the half- or unexpanded condition, but disappears when the 

 branchia is fully unfolded. The anus, which is often widely opened, 

 is lateral in position. The tint of the coloured spots and bands 

 of the back, cephalic expansion, and lateral appendages is subject to 

 great variations in different individuals ; it ranges from brown to 

 black, and varies in intensity ; it may also present dull spots ; the 

 markings are however always well defined on old specimens. 



MoUuscoida. 



Tunicata of the ' Challenger ' Expedition, &c.* — Dr. W. A. 



Herdman has undertaken the descriptions of this collection, com- 

 prising from 150 to 200 species, the majority of which are new. The 

 paper now published is a preliminary report on two families of the 

 Ascidiae simplices, viz. the Ascidiadas and the Clavelinidge. 



The Ascidiadae are synonymous with Forbes' Ascidia, and include 

 those simple Ascidians which are as a rule (though not without ex- 

 ception) externally characterized by an 8-lobed branchial and a 

 6-lobed atrial aperture, as distinguished from the Cynthiadas (both 

 4-lobed) and the Molgulidte (branchial 6-lobed and atrial 4-lobed). 

 Of the live previously known genera, two (JRlwpnlcea and Rhodosoma) 

 are not represented ; but there is a new genus, Abyssascidia, obtained 

 at the great depth of 2600 fathoms. The four genera include seventeen 

 species, fifteen of which are new. 



Of the Clavelinidfe — a small group of social Ascidians — there is 

 a new genus, Ecteinascidia (three species), and two new species of 

 Clavelina. Dr. Herdman gives his reasons for considering that, 

 instead of occupying a position intermediate between the simple and 

 compound Ascidians, the group should be included in the former. 



Dr. Herdman also describes and figures t thirteen species (six 

 nov. sp.) of Ascidiae dredged during the last three summers on the 

 west coast of Scotland, in Loch Long and in Lamlash Bay, Arran. 



Budding of Pyrosoma. j — The question of the origin of the peri- 

 branchial spaces or lateral atria in the bud of this Tunicate, which 

 baffled the researches of Huxley and Lacaze-Duthiers, has been 

 decided by L. Joliet in favour of the mesodermic tissue which occu- 

 pies the interval between the endostyle and the test. 



In a transverse section of a sufficiently young bud, this tissue 

 may be found to contain the generative rudiments, the nerve-ganglion, 

 and two oval masses of cells which represent the walls of the peri- 

 branchial spaces ; the same result is arrived at by examination of a 

 median longitudinal section. Even in the adult the wall of the 

 branchial sac on the neural side is connected with the epidermis by a 

 loose tissue which is evidently mesodermic, while on the opposite 

 side, between the endostyle and the test, lies an analogous tissue 

 traversed by blood-vessels, and becoming a regular lamina at the 

 posterior side. Thus the endostyle is accompanied by a tissue 

 derived from the mesoderm which develops in the bud into the 



* Proc. R. Soc. Edin., x. (1880) pp. 458-72, 714-26. 



t Journ. Linn. Soc. Lond. (Zool.), xv. pp. 274-90 (tj pis. and 3 figs.). 



X Comj)te3 Rendus, xcii. (1881) pp. 473-5. 



