Royal Physical Society. 413 



the anomalous character of their structure, I believe that their 

 systematic position can alone be determined by accurate ob- 

 servations on the development of the embryo, and careful dis- 

 sections of the animals themselves. The distribution of these 

 parasites in Europe appears to be very wide, especially that of 

 Peltogaster carcini. Specimens of this species have been ob- 

 tained from the coasts of the Mediterranean, Crimea, Norway, 

 and from the " Black Banks" in the North Sea. Professor 

 Bell was the first to notice it on our coast ; and I have also to 

 record it as a British parasite. 



The Peltogaster paguri does not appear to be so exten- 

 sively distributed as the former species. Rathke, who first 

 described it, obtained his specimens from the Norwegian coast ; 

 Kroyer appears to have met with it in the Kattegat. As this 

 species has never been recorded as a British animal till now, 

 I shall confine the remainder of my remarks to the consider- 

 ation of it. 



Habits and External Characters. — The invariable position 

 of this parasite is on the left side of the abdomen of the crab 

 it infests, immediately below the false feet. They may be 

 found from spring to harvest, and during the early part of 

 summer are usually filled with eggs. The size varies with 

 the crab on which it lives. The colour is orange-yellow, 

 tinged with copper-green. All organs of sense are absent, 

 and it presents the appearance of a well-filled sack tied at the 

 mouth. It forms a moderately-curved oval, terminated in a 

 short snout, which opens into the cavity of the body. This 

 opening is crenated, and always directed to the thorax of the 

 crab. I have observed in this parasite a remarkable systolic 

 and diastolic-like movement of the whole body, but the ob- 

 ject of which I cannot as yet explain. The disc by which 

 the Peltogaster attaches itself is of a horny consistence, and 

 is situated in the middle of the body. It is star-like in form ; 

 and by means of it these parasites are so deeply rooted into 

 the body of the crab in which they live, that they are unable 

 ever to leave their position. While speaking of the disc, I may 

 mention a fact in connection with it, for which I am indebted 

 to Dr T. S. "Wright for directing my attention. It is this, that 

 through the opening which exists in the centre of the disc 



