ZOOLOGr AND BOTANY, MICROSCOPY, ETC. 73 



cells on the surface are arranged in a special or characteristic manner, 

 which is described in detail. 



Even in the latest stages of segmentation one can only distinguish 

 two layers, an ectoderm formed by the micromeres and some of the 

 macromeres ; and a meso-endoderm represented by cells varying in size 

 and form, but all derived from the macromeres; -after the appearance 

 of the medullary groove the meso- and endoderm become differentiated ; 

 by a reference to his figures, the author demonstrates that the large 

 cells at the hinder end of the body undergo a retrograde development ; 

 the ectoderm thickens, and gives rise to the ventral pads or medullary 

 plates ; at the top of the cephalic tubercle it thickens, and becomes 

 composed of several layers, so as to give rise to the " sincipital 

 plate " ; the endoderm begins to form the oesophagus. 



The nervous system is developed from the ventral pads or plates, 

 which are homologous to the similar organs described in other 

 Annelids, and which give rise to the ventral ganglionic chain, and 

 from the sincipital plate which forms the cephalic ganglia. The 

 history of these is given in detail. The cerebral commissure appears 

 later than in the Chsetopoda, and is not complete till the time when 

 the embryos are just about to escape, and it consists of nerve-cells ; 

 the cephalic ganglion after its separation from the ectoderm is com- 

 posed of a continuous cellular mass ; in the adult there are two pairs 

 of ganglia, but this structure is a secondary and not a primitive one, 

 for it is produced in a relatively late stage, and only just precedes 

 the formation of the mouth. 



On the whole, the formation of the coelom is as in other Annelids ; 

 the differences in the mode of formation of the dissepiments and their 

 relation to the external membranes is discussed ; as to the sucker, 

 Prof. Salensky can only confirm the results of other observers as to its 

 being a modification of the posterior metameres of the embryo. The 

 endoderm is at first a compact mass of cells, and the enteron is, com- 

 pared with that of other Hirudinea, formed late ; the stomo- and 

 proctodoea are short, and the former only gives rise to the lips ; the 

 constrictions of the caeca correspond to the dissepiments. 



Priapulus caudatus and Halicryptus spinulosus.* — Dr. W. Apel, 

 after some remarks on methods of preservation, and observations on 

 the living animal, proceeds to describe in detail the structure of these 

 two Gephyrean worms. 



As in P. hicaudatus, the cuticle consists of two sharply dis- 

 tinguished layers, the outer of which is homogeneous and structure- 

 less, while the lower consists of fine lamellas marked off by two 

 systems of lines. The papillae of Priapulus and those on the proboscis 

 of Halicryptus have the same essential structure as, and are doubtless 

 homologous with those of the trunk of the latter ; the only difference 

 is that the former have an orifice at their tip. The musculature of 

 P. caudatus and Halicryptus is similarly arranged as in P. hicaudatus, 

 as described by Horst. The wall of the caudal appendage of 

 Priapulus has the same structure as the body- wall, save that the circular 



* Zeitschr. f. Wiss. Zool, xli. (1885) pp. 459-529 (3 pis.). 



