406 
BULLETIN OF THE UNITED STATES FISH COMMISSION. 
I have not been able to determine how 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 vary 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 from the normal. 
Fig. 67, PI. 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 Venus , ( 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 Venus. They frequently 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 VASCULAR SYSTEM. 
The heart, the chief organ of the circulatory system, is in the majority of cases 
situated on the dorsal part of the body and is greatly elongated from 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. 
l xxx vt, 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 ventrally; 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, Ho. 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 Nucula , 
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 Amphi- 
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. 
