224 NATURAL SCIENCE. Sept.. 



it is more likely to disguise than to reveal kinships. Dr. Brooks 

 considers it a corollary of growth by cell-reproduction, but to whatever 

 biological cause it may be due, too much importance has been 

 attached to it in the framing of phylogeny. 



The ancestor of Appendicularia he pictures as a " simple, minute, 

 unsegmented chordate animal, leading a free locomotor pelagic life, 

 and subsisting on the micro-organisms of the ocean. It had an 

 elongated, unsegmented body stiffened by an axial, unpaired, unseg- 

 mented notochord like that oi Amphioxus, Appendiculana, andthe ascidian 

 larva : a simple elongated dorsal nervous system : and an elongated, 

 ventral digestive tube without pharyngeal clefts." This tube was 

 distended and ciliated, and the water with the micro-organisms in it 

 was swept through by ciliary motion, not by contractions of the body- 

 wall. 



In process of time there came slime cells to retain the food, and 

 Natural Selection preserved and accentuated the slime cells in the 

 anterior region. The free current of water passing through the 

 animal at first swept in new organisms, and swept even any partially 

 digested organisms. However it might have appeared, a pharyngeal 

 cleft that would allow this free stream of water to pass out, leaving 

 behind the slime-bound food, would be an advantage too great not 

 to be selected, and one gill cleft being established. Dr. Brooks 

 believes that by a " law of growth " it would become paired. 



Thus he finds in the primitive gill clefts mechanical advantages 

 for nutrition quite unconnected with respiratory or excretory functions. 

 The velum, the endostyle, and the peripharyngeal bands similarly 

 were food-catching mechanisms. 



He traces out stages by which fixed Ascidians might easily have 

 been derived from a free-living form like Appendiciilana. 



In the next section Dr. Brooks criticises the Annelidan 

 theory very closely ; but we imagine that, however fertile they have 

 been in suggestions, and full as they are of brilliant morphological 

 detail, not many naturalists support the main thesis of Dr. Dohrn's 

 well-known studies ; and for that reason it is hardly necessary to 

 refer in detail to the present criticism. Still less to the associated 

 criticism of the hypothesis that the Tunicates are degenerate 

 Selachians. 



What is more interesting is the view Dr. Brooks takes of the 

 coelome. Since Hertwig first insisted on the importance of this 

 method of development of the body-cavity, it has been accspted 

 as final that the chordate body-cavity is an enterocoele — that is to say, 

 that it is developed from series of pairs of gut-pouches, and that the 

 simplicity of Appendicidavia cannot be primitive, as its ancestors once 

 possessed a segmented enterocoele. Seeliger gives a minute account 

 of the origin of the mesoderm in Clavelina, and states that it comes 

 from two totally unsegmented rows of cells, while the body-cavity is 

 really a primary segmentation cavity. DavidofF, working on Distaplia, 



