ZOOLOGY AND BOTANY, MICKOSCOPY, ETC. 463 



mesodermal cells ; musculo-epitlielial structures give rise in both to 

 the musculature of the water- vascular system, or, in other words, three 

 muscular fibrils may, after treatment with osmic acid, be seen to be 

 connected with the separate epithelial cells which invest the caecal 

 processes of the rudimentary water-vascular system. On this point 

 the author insists in considerable detail,, and concludes by stating that 

 in the fore-gut of Tornaria one finds just the same fine fibrils of 

 circular muscles which are so characteristic of the typical Echino- 

 derm larva, and, as in them, that part of the gut is the only region 

 which has a proper musculature. 



The order in which the steps of metamorphosis are passed through 

 is the same for the two sets of animals ; for example, the longitudinal 

 ciliated baud arises before the anal circlet. A young Tornaria might 

 be easily confounded with a young Auricularia, but never with a Trocho- 

 sphcera or Actinotrocha. The supposition that there is a close affinity 

 between Tornaria and the JEchinopcedia (or larval Echinoderms) is only 

 possible on the view that there is nothing essentially diiferent in their 

 plan of structure. Nor is there ; bilateral symmetry is typical of the 

 Echinoderm larva, and persists throughout life in Balanoglossus. The 

 water-vascular system, which is so eminently characteristic of the 

 Echinodermata, is represented in Balanoglossus by the proboscis sac, 

 which opens by a dorsal pore, and which, instead of growing out into 

 radial prolongations, remains at a lower stage of development. If 

 this view be correct, the so-called proboscis may be regarded as a 

 single, conical, ambulacral tentacle ; and it is not to be forgotten that 

 in Synapta, which is a true Echinoderm, not only the ambulacral 

 caecal sacs, but also the longitudinal trunks are lost. The peritoneal 

 system would also seem to be represented in Balanoglossus, and is 

 lined by just the same kind of membrane as in the Holothurians, and 

 the circulatory system in these Echinoderms is very similar to what 

 is seen in Balanoglossus. The author looks upon the gills as rudi- 

 mentary water-vessels delayed in development and undergoing 

 vegetative repetition. The absence of calcareous matter from the 

 integument of the " worm " is a notable point of ditference ; but, on 

 the other hand, the nervous system is very similar. 



As a result of his studies, the author proposes to form the " type " 

 Ambulacraria, with the two sub-types of Madiata and Bilateralia. 

 The former will include forms with a radial plan of structure 

 developed on the bilateral embryo, with a greatly developed cutis, 

 containing calcareous deposits and a complicated water-vascular 

 system, arranged in a radial fashion. The latter retains definitely 

 its bilateral symmetry, has no calcareous deposits, and has the water- 

 vascular system less elaborately developed. 



Echinodermata. 



Arctic Echinodermata.* — Professor Martin Duncan and Mr. W. 



Percy Sladen have published (aided by a Government grant) an exact 



account of the thirty species found, chiefly by the officers of the 



' Alert ' and ' Discovery,' between 79° 20' N. lat. to 82° 27' N. lat. In 



* 4to, London, 1881, 82 pp. (6 pis.). 



2 I 2 



