354 



have homological parts ; which is entirely disregarded in this 

 classification. 



Mr. Desor replied, that this classification he presented as new 

 in comparison with Cuvier's arrangement, which was still the 

 standard of our text-hooks ; and in this respect he maintained, 

 that Prof. Vogt's classification, as a text-book, might be consid- 

 ered new. 



Prof. Agassiz made some remarks on the little bodies 

 seen on Hydra, which have been described as parasites, by 

 several authors. 



These parasites at times leave the Hydra, and swim free, 

 changing their form in a remarkable degree. In addition to the 

 internal ring, he was able to trace rays going from the hooks to 

 the margin, divided into numerous branches, and also rays pro- 

 ceeding towards the centre from this ring; the margin has a 

 fringed undulating edge, under the tentacles. By feeding them 

 with colors, he was able to see that the internal folds are the margin 

 of a mouth, as in Rhacostoma, so that these parasites on Hydra 

 are diminutive Medusse. In the egg of Hydra, he had been able 

 to trace all the forms from a segmented yolk to these parasites ; 

 the fresh water Hydra is the Polypoid form of Medusse, while 

 these parasites are the Medusoid form. In the American Hydra, 

 as in the European, there are two types, the Iroion and the 

 green. The American green Hydra, unlike the European, has 

 the power of extending its body in a remarkable degree ; he 

 would call the American Hydra, gracilis. Our broivn Hydra 

 has very short tentacles, while the European has long ones ; 

 he would call the former Hydra carnea. 



Dr. Wyman remarked that he had noticed numerous parallel 

 canals running not only from, but between the " hooks " of these 

 parasites ; these had also been noticed by Prof. Agassiz. The 

 spots described by Prof. Agassiz as gastric cavities he had seen 

 rapidly change their size, even contract and disappear, as is the 

 case with the spots seen in Baeccilloscina. 



Prof. Agassiz, in continuation of the subject of the ramifica- 

 tions of the water tubes in Scutella and Medusae, observed that 

 in the walled Echinoderms, in those with no lateral expansions. 



