FUNDAMENTAL RELATIONS OF ANIMALS 79 



young are either polyplike or resemble more immediately the type 

 of their class. Few multiply in a direct, progressive development. 

 As to Echinoderms, they have for a long time almost entirely es- 

 caped the attention of Embryologists, but lately J. Miiller^"^ has pub- 

 lished a series of most important investigations upon this class, dis- 

 closing a wonderful diversity in the mode of their development, not 

 only in the different orders of the class, but even in different genera 

 of the same family. The larvas of many have a close resemblance to 

 diminutive Ctenophorae and may be homologized with this type of 

 Acalephs. 



As I shall hereafter refer frequently to the leading divisions of the 

 animal kingdom, I ought to state here that I do not adopt some of 

 the changes which have been proposed lately in the limitation of 

 the classes and which seem to have been pretty generally received 

 with favor. The undivided type of Radiata appears to me as one of 

 the most natural branches of the animal kingdom, and I consider 

 its subdivision into Coelenterata and Echinodermata as an exaggera- 

 tion of the anatomical differences observed between them. As far as 

 the plan of their structure is concerned they do not differ at all, and 

 that structure is throughout homological. In this branch I recognize 

 only three classes. Polypi, Acalephce, and Echinodermata. The chief 

 differences between the two first lies in the radiating partitions of 

 the main cavity of the Polypi supporting the reproductive organs; 

 moreover, the digestive cavity in this class consists of an inward 

 fold of the upper aperture of the common sac of the body, while 

 in Acalephs there exist radiating tubes, at least in the proles me- 

 dusina, which extend to the margin of the body where they anasto- 

 moze, and the digestive cavity is hollowed out of the gelatinous mass 

 of the body. This is equally true of the Hydroids, the Medusae 

 proper, and the Ctenophoras; but nothing of the kind is observed 

 among Polypi. Siphonophoras, whether their proles mednsina be- 

 comes free or not, and Hydroids agree in having, in the proles 

 medusina, simple radiating tubes, uniting into a single circular tube 

 around the margin of the bell-shaped disk. These two groups con- 

 stitute together one natural order, in contradistinction from the 

 Covered-eyed Medusae, -whose radiating tubes ramify towards the 

 margin and form a complicated net of anastomoses. Morphologically, 



i« [Johannes Peter Muller, 1801-1858.] 





