406 BULLETIN OF THE UNITED STATES FISH COMMISSION. 



I have not been able to tletermiue bow many times a mother cell divides in forming 

 spermatozoa, for the cells are all rounded and give no evidence of their divisions, as 

 they do in the testes of many animals. A duct of the testis containing spermatozoa is 

 shown at d. The ducts of both testis and ovary are composed of slightly columnar^ 

 ciliated cells. In the wall of the duct of the testis is shown a single deeply stained 

 cell, which is evidently a gland cell. 



In the mother sperm cells of Mytilus the chromatin is arranged in a crescent-shaped 

 mass at one side of the nucleus. 



The spermatozoa varj^ greatly in different forms, both in size and general shape. 

 In a single follicle of the gland of any individual, also, they are of various shapes, often 

 very markedly different fi'om the normal. 



Fig. 67, PL Lxxxviii, represents a very few spermatozoa illustrating these points. 

 The " tails " in all these cases are more than twice the length represented in the figure. 

 The sharply pointed spermatozoan " head " of Yoldia is shown (a) to be similar to that 

 of Venvs, {b) in that both are elongated and conical in outline. The latter, however, 

 are always bent. In a very large number of cases the tail proceeds from the narrower 

 end of the head (c). The spermatozoon of Pecten is represented at d. It is actually 

 much shorter than that of Veyius. They fr'equently show a form similar to/. The 

 spermatozoon of the oyster (e) has a nearly spherical head, which gradually tapers off 

 into the tail. If spermatozoa are characteristic even of species, as has been suggested, 

 it may be of interest that those of Anomia are very similar to those of the oyster, 

 since Pelseneer does not regard these forms as being in any way closely related. 



THE VASCIILAK SYSTEM. 



The heart, the chief orgau of the circulatory system, is in the majority of cases 

 situated on the dorsal part of the body and is greatly elongated fi^om before back- 

 ward. It consists of a muscular ventricle and two lateral, generally more delicate, 

 auricles opening into it, one on either side. The organ lies in an extensive pericar- 

 dium. The usual relations in position between these parts may be seen in Fig. 45, PI. 

 Lxxxvi, where ven represents the ventricle, au the auricle, and p the pericardium . In a 

 few cases, as in the oyster, the heart has changed from its usual position, and its long 

 axis is dorso- ventral. In Fig. 97, PI. xciv, the organ may be seen represented as lying 

 beneath the pericardium, just in front of the adductor muscle. The ventricle is most 

 dorsal, and the two auricles open into it ven tr ally; these are pigmented ; they receive 

 blood from the gills. 



As a rule the ventricle of the heart is traversed by the rectum, but this does not 

 occur in Nucula, Area., Anomia., Meleagrina, Ostrea, or Teredo. Its position in the 

 ventricle of Pecten is shown in Fig. 45, r. In Solenomya the rectum is close to the 

 ventral wall of the ventricle, instead of being connected with the dorsal wall, as is 

 generally the case. (Pelseneer, ISTo. 17.) In the forms in which the ventricle is not 

 traversed by the rectum, Nucula, Area., and perhaps Anomia are primitive forms, while 

 Ostrea and Teredo are among the most specialized of lamellibranchs. But in Xncula, 

 Area, and Anomia the ventricle is dorsal to the rectum, while in Meleagrina, Ostrea, 

 and Teredo it is ventral to it. In certain primitive mollusks {Cephalopoda and Aniphi- 

 neura), the ventricle of the heart is dorsal to the rectum, as in Nucula, etc., and it is 

 probable that this is the primitive condition. 



