ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 759 



sistonce of the prsB-oral ring of cilia, probably of the head-kidneys, as 

 well as ill the general characters of the enteric canal, the adult Dino- 

 philus may be considered to remain in a condition which, is practically 

 that of a tr()c]ios2)liere. 



Eotifera.*— Dr. C. T. Hudson has issued a supplement to the well- 

 known work by himself and Mr. Gosse on the Eotifera. He gives in it 

 a description of every known foreign species, as well as of such British 

 ones as have been discovered since 188G. More than 150 species are 

 here described, and more than 40 doubtful or imperfectly described 

 forms are briefly discussed and occasionally illustrated. 



Echinodermata. 



Homologies within the Echinoderm-phylum-t — Dr. E. Semon c(im- 

 pares the several organs of the representatives of different orders of the 

 Echinodermata with one another. Some of them he regards as com- 

 pletely homologous — such are the enteric sjstem, the enterocoel, the 

 water-vascular, and the nervous systems. Other structures are merely 

 analogous or homoplastic, as arc, for example, many jiarts of the skeletal 

 apparatus. Other structures are neither hcmologous nor analogous, for 

 the common organ from which they were derived was of so inditferent a 

 nature that from it there liave been independently developed in the 

 various classes organs \\hicli are sometimes similar and sometimes very 

 different. Under this head may be placed the musculature; it is in all 

 cases derived from the typical dermomuscular tube, but presents great 

 differences in Holothurians, Starfishes, and Sea-urchins ; here, too, come 

 parts of the water- vascular apjjaratus and of the nervous system of 

 Holothurians. If we suppose that the primary tentacles of Holothurians 

 correspond to those of the other classes, the water-vessels of the body 

 and the nerves of Holothurians have only a general and not a special 

 homology with those of the other classes; if, on the other hand, we 

 regard the secondary tentacles of Holothurians as strictly comparable to 

 those of the other classes, then the primary tentacles have only a general 

 homology. 



In the present state of our knowledge it is impossible to decide how 

 far th^ blood-vascular systems of the various classes are generally or 

 specially homologous or merely homoplastic ; the dorsal organ has 

 probably a general homology. 



Comparative anatomy indicates that the classes of the Echinodermata 

 were certainly derived from a common stem-grou]), but this, in its 

 general structure, exhibited somewhat indifferent characters, and the 

 various classes have diverged in their development from this point of 

 origin. The history of development appears to support this view. The 

 author congratulates liimself that the views he has already expressed 

 have been confirmed by the investigations of the Drs. Sarasin. 



The simplified Synnpta of the Sarasins is not very different to the 

 pentactula-like stem-form of the author; bnt they are not quite correct 

 ill speaking of this as a Holoihuria-stage. 



Neumayr is also inclined to recognize a very early divergence of 

 the classes from simple stem-forms, but he regards the Echinoids and 



* 'The Eotifera or Wheel Animalcules,' by C. T. Hudson, assisted by P. H. 

 Gosse. Supplement, London, 1889, 64 pp., 4 pis. 

 t Morphol. Jahrb., xv. (1889) pp. 253-307. 



