778 SUMMARY OF CURRENT RESEARCHES RELATING TO 



on cither side of their gill-organ ; Doliolkho have the holes hotter 

 developed and more numerous; the Didesmids have three rows, and 

 the Leptoclinidffl four. In higher types the gill is larger and is placed 

 heside the intestine. In the Thaliacca tho respiratory organ is very 

 siniplo, and without papilla); the forms in which the gill is simple 

 may he grouped together as the Aplousobranchiata. The second 

 natural sub-class is that of the Phlehohranchiata, in which the gill is 

 provided with longitudinal vessels. The third and last, or that of 

 the Caducichordata, have longitudinal folds on the gills, for which 

 reason they are denominated the Stolidobranchiata. The progressive 

 complication of the gill is stated to correspond to an increase in tho 

 differentiation of the whole Ascidian organism, and the proposed 

 classification is not therefore to he supposed to depend on a single 

 character. 



Histology of Digestive Tract of simple Ascidians.* — M. L. 

 Eoule in this investigation has chiefly studied the Cynthiidse ; fn tho 

 walls of their digestive tract he finds special tubular elements, which 

 are very small (10 to 15 /a in diameter) and very numerous. They 

 jire absent from the oesophageal region, and are most often grouped 

 around blood-spaces. Their wall is very simple, and consists of a 

 single layer of epithelium, formed of small cubical cells, with a large 

 nucleus; in their cavity there are more or less fine amorphous 

 elements. Terminal dilatations are occasionally to be seen, and these 

 bring to mind the renal capsules of Bowman. There appears to be a 

 similarity in form and structure, as there is doubtless also in function, 

 between these tubes and the renal organs of vertebrates ; but it must 

 be borne in mind that they have quite a different developmental 

 history. 



New Diplosoma.j — M. F. Lahille describes Diplosoma Kcehleri, a 

 new species of Synascidian found at Guernsey and Eoscoff, and gives 

 some details as to its structure. The statement of Delia Valle that 

 fecundation in the Diplosomidae is effected by the aid of a pore 

 which opens to allow of the passage of the spermatozoa, and again 

 closes, is shown to be incorrect. 



y- Brachiopoda. 



Anatomy of Brachiopoda Inarticulata.J — M. L. Joubin has 

 chiefly studied Crania, which is found in abundance at Banyuls, and 

 compares it with the other two known genera of this group, Lingula 

 and Discina. The lower differs from the upper valve in the structure 

 of the perforations, the disposition of the calcareous layers, the plexus 

 of fine canaliculi, and by the calcareous incrustation of the layers of 

 cartilage which are found in it. The mantle is formed of a delicate 

 cartilaginous membrane covered on either side by a layer of cells ; 

 those which are applied against the shell form a kind of plexus whence 

 arise tubes which invest the canals with which the shell is tunnelled. 



* Coraptcs Renclus, cii. (1886) pp. 1503-6. t Ibid., pp. 416-8. 



% Arch. Zool. Exper. et Gc'n., iv. (18S6 pp. 161-303 (9 pis). 



