88 SUMMARY OF CURRENT RESEARCHES RELATING TO 



flagelLa become developed and work actively and rhythmically at first. 

 The stomodaBum after a time disappears, to make way for the definite 

 mouth, which appears at the same spot as the external stomodfeal orifice ; 

 its presence allows one to distinguish a ventral from a dorsal surface. Just 

 below the stomodteum the ectoderm becomes again depressed in the middle 

 line, and forms a narrow, short, blind tubule, which is directed obliquely 

 inwards and do'.vnv\ards; its inner wall is covered by fine cilia, which 

 appear to have a very complicated motor rhythm. 



In the third chaj)ter the author deals with the youngest larvge, and in 

 the fourth with the development of the various tir^sues ; the fifth is a 

 contribution to the theory of the neuromuscular system. 



The larva of Lopadorlnjnchus in its simplest form — that is, when it 

 commences to lead an independent free life, but is not yet laden with the 

 extensive and numerous rudiments of the annelid organism — is an almost 

 spherical body, sharply divided into an upper and a lower hemisphere. On 

 the lower one lies the mouth formed by a stomod^eum. The endoderm, 

 and, for the greater part of its extent, the ectoderm, is a single uniform 

 layer ; between the two are a few contractile cells. In the prototroch, at 

 the boundary of the two hemispheres, there is a special and proportionately 

 rich organization^there is the locomotor organ, a simple ring of large cells, 

 with strong cilia, with which is connected an upper and a lower row of 

 smaller ciliated cells, a nervous system, and a circular muscle. The 

 nervous system is composed of regularly disposed fibres and cells ; the 

 chief part of the fibres forms a closed ring, and the fibres are the processes 

 of two kinds of ganglia, one of which is called automatic and the other 

 reflex. At the point where, later on, the cephalic ganglion arises there 

 are a few ganglionic cells, which are clearly not constituents of the larval 

 system, in the way of formation. 



The central nervous system of the larva of Lnpadorhynchus may be 

 regarded as consisting of a fairly diffused nervous plexus, the processes 

 of which partly pass into other tissues, and so form the peripheral system. 

 But, within this plexus, a further centralization is brought about, the 

 closed system of the nervous ring being the controller of the whole system. 

 t*uch a system is, as a permanent arrangement, found among the craspedote 

 Medusfe, and there are essential resemblances between the two. The topo- 

 graphical relations of the margin of the umbrella with the velum, and of 

 the prototroch with the general body are the same, and both are the chief 

 organs of locomotion. Their contractile elements are, however, very 

 different, in correspondence with the difference in their structure. If we 

 suppose that the j)rototroch of the annelid is the homologue of the mai'gin 

 of the umbrella and velum of the medusa, then the Annelid-larva might be 

 classified in or near the Hydi'O-medusae, and the stem-form of the annelids 

 is to be souglit for in a form which stands much nearer the craspedote 

 Medusae than the hypothetical organism which Balfour took to be the 

 originator of Pilidium, Trochosphsera, Tornaria, Actinoirucha, and the larvae 

 of Echinoderms and Brachiopoda. 



Organogeny of the Hirudinea.* — Herr J. Nusbaum has studied in 

 Clepsine complanuta Jav. the later development of the Hirudinea. After 

 referring to unfavourable technique the opinion of Hoffmann as to the 

 absence of proper germinal layers, Nusbaum discusses, in the first jdace, 

 (1) the develoi)ment of the hodij-cavity and of the muscular and connective 

 tissue. As in higher worms, each mesodermic band divides into 23 somites, 

 in each of which a cavity appears. The replacement of the somite spaces 



* Arcli. Slav, dc Biol., i. (18SC) pp. 320 40 (4 pis.) and pp. 5:J9-56. 



