ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 229 



nucleolus, appear. The cephalic nodule is now formed, as a small 

 cup-shaped disk, attached to the surface of the nucleus. As in the 

 Vertebrata, the spermatic filaments are developed at the expense 

 of the spermatoblasts, but the cephalic nodule, which in all other 

 animals has an important function, is here only secondary. A little 

 later the spermatozoid is found to consist of three segments — a 

 cephalic, which incloses the spermatoblast and its nucleus, a median 

 segment which is scarcely visible, and a caudal segment or filament. 

 Later on, the nucleus becomes ovoid in shape, with its long axis in an 

 antero-posterior direction ; after it has elongated, its hinder extremity 

 separates from the cell-body of the spermatoblast, and it finally leaves 

 the cell. The flagellum becomes of proportionately great size. 

 Eighty to one hundred spermatic filaments are united into bundles, 

 which are placed in the grooves of the epithelial cells which line the 

 walls of the tubes. Isolated spermatozoa have only been found in 

 the oviducts of the female. 



With the exception of its cephalic nodule, the spermatozoon of a 

 hedriophthalmate crustacean has very much the same history as that 

 of the Selachians, and it is to be noted that these spermatozoa exhibit 

 a more complete type than those of the Podophthalmata, inasmuch as 

 in the latter they may be reduced to the single cephalic segment. 



Vermes. 



Structure and Division of Ctenodrilus monostylos.* — M. Zep- 

 pelin gives a full account of this new species of marine annelid. It 

 is about 3 or 4 mm. long, and 0*2 wide, and consists of 20-25 well- 

 marked segments, and is of a yellowish-brown colour. It is remark- 

 able for the possession of a protrusible proboscis which is quite 

 independent of the enteric canal. The only pair of segmental organs 

 is found in the head. The buccal cleft is ciliated, as are also the 

 oesophagus and the rectum. All the segments but the last have 

 setigerous sacs, with two or three sette apiece. They move very 

 slowly. Sexual reproduction has not yet been observed, but only 

 transverse division. The habitat of the worm is not known, the 

 specimens examined having been found in an aquarium at Freiburg. 



The cuticle is thin and homogeneous, the hypoderm thick and 

 made up of polygonal cells with scattered pigment-spots. The 

 musculature is of a very primitive character, the dermomuscular 

 tube consisting of a simple layer of longitudinal fibres which extend 

 uninterruptedly to the end of the body ; in this point G. monostylos 

 has a striking resemblance to Polygordius. Although metamerism is 

 very distinctly expressed externally, it is not so well marked inter- 

 nally as in most forms, the enteron, for example, not being constricted 

 by the dissepiments. The setse are very regularly distributed over 

 the body, and are either thin and sharp, or stronger and shorter ; the 

 two last metameres which are not so well differentiated as the rest, 

 have, as a rule, no setse. The enteric canal is a little longer than the 



* Zeitschr. f. Wiss. Zool., xxxix. (1883) pp. 615-52 (2 pis.). 



