1873.] 181 [McCrady. 



extending the whole length; the mouth by which it is fastened to the 

 jelly-fish (to the inner wall in the upper part of the long furrow, near 

 the eye-speck) occupying the whole of the anterior part. This mouth 

 can be closed, extended to a point, and when inserted in the sub- 

 stance of the jelly-fish, it is expanded again like the mouth of a trum- 

 pet, and the worm is firmly fastened." These worms, he tells us, are 

 sluggish in their movements, and exhibit only slow contractions of 

 their bodies when detached, though they will live several days after 

 being separated. He cannot refer them to any of the genera de- 

 scribed, though he regarded them as resembling a leech more than 

 anything else. 



It is curious that the only jelly-fish I have observed in the pond 

 where these infested oysters were bred, was the Mnemiopsis littoralis, 

 whose development I traced in 1857, and described under the name 

 of Bolina. But they were taken in the month of April, and no par- 

 asites of any kind were observed upon them; nor have I observed 

 Bucephalus cuculus in any later stage of growth than that described 

 in this paper. The great size of A. Agassiz' worm (its length vary- 

 ing from an inch to an inch and a half), and its five white lines, are 

 very unlike the parasite of the oyster. But the Cercariae, to which 

 group of larval forms Bucephalus belongs, appear to be all young of 

 Trematoda, and in our ignorance it is well to bring into comparison 

 with each other all the facts we can collect. 



GENERAL REMARKS. 



The remarkable fact that in the case of Bucephalus the " sporo- 

 cyst " is a branching stem, in which is formed a continuous canal, 

 has, so far as I am aware, no analogy, except among the Hydroid 

 Medusae, and some Polyzoa and Tunicata. Among the Hydroids, 

 the ciliated embryo, as in Melicertum, according to A. Agassiz' 

 observations, sometimes simply elongates into a worm-like shape, 

 enlarged at one extremity, and enclosing a simple cavity of nearly 

 the same form as the embryo. By the enlarged extremity these em- 

 bryos attach themselves, and continue their growth, sometimes even 

 put out lateral buds before the formation of a polyp-head. Here, 

 therefore, we have an embryo in the form of a branching stem 

 with central canal, and soon clothing itself with stiff chitinous 

 sheath. In other Sertularians, and the case which I recall was, I 

 think, an Obelia, a colony may, according to my observations, 



