ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 225 



between tlie two lobes of the velum, wlucb also appear as two ridged 

 arcs of large ciliated cells. 



The cephalic ganglia appear independent of and long before the 

 pedal, in the form of two ectoderm plates in front of the velum, and 

 separated by the above-mentioned anterior ciliated ridge. These 

 thicken and sink in to form at once the ganglia and the eyes, which 

 remain always in close association. The ganglia consist at a median 

 stage of two blind rods, with a narrow lumen and external opening ; the 

 blind ends form processes which meet and grow together, and mean- 

 while the cerebral rods have become solid. The separation of the 

 ganglia occurs at a late stage. 



The auditory sacs appear before the pedal ganglia as small ecto- 

 dermal pits on the margin of the foot, becoming afterwards constricted 

 off into sacs. Soon afterwards, at each side of the ciliated ridge of 

 the foot, two ectoderm thickenings appear ; they are slowly modified, 

 and at length become neural plates of several layers, which are aftei'- 

 wards separated from the ectoderm and grown round by mesoderm. 



Two distinct large glands are formed in the foot ; the posterior, 

 formerly described by Lacaze-Duthiers in the adult Vermetus, 

 appears as a sac-like deepening of the ectoderm, and occupies the 

 whole posterior portion of the foot, the anterior consisting of a com- 

 pact cell-mass with a cylindrical duct of some length. Its history is 

 uncertain. 



The mesoderm remains partly unsplit behind the foot and this 

 portion forms the rudiment of the musculus columellaris, while :; con- 

 tinuation of it seems to form the rudiment of the j^cricardium which 

 appears at an early stage, on the right side of the embryo, as a 

 thin mesoderm sheath, which soon splits to form a cavity obviously 

 homologous with the body-cavity. In the posterior corner of the 

 cavity the splanchnic layer becomes raised from the endoderm, and 

 the cavity of the heart is thus formed between the two layers. The 

 oesophagus and radula-sac are formed from the inbent margins of the 

 blastopore ; the hind-gut appears as an ectodermal plate, which projects 

 conically, becomes hollow, and acquires an anal opening. 



Prof. Salensky notes how the development demonstrates the 

 homology existing between certain portions of molluscan and annelid 

 nervous systems, thus the molluscan cephalic and j^edal ganglia are 

 the equivalents respectively of the annelid supra-cesophageal and first 

 ventral ganglia. 



Anatomy of the Marine Rhipidoglossata.* — In his second essay 

 on these Mollusca, Dr. B. Haller deals with the texture of the 

 central nervous system and its investments. The methods of ex- 

 amination adopted were to remove the nervous system from the living 

 animal, and to harden it in pure alcohol, chromic acid solution, or 

 hyperosmic acid ; these suited various tissues in different degrees ; for 

 isolation a mixture of glycerin, acetic acid, and distilled water was 

 used. 



1. The ganglionic cells ; the author finds that the process arises 



* Moiphol. Jahrb., xi. (1885) pp. 321-430 (8 pis.). 

 Ser. 2.— Vol. VI. O 



