108 REPORT— 1873. 



thought he could trace two hue filaments proceeding from it and running down, one 

 towards the right and the other towards tlie left side of the alimentary canal ; but 

 he was not able to follow them for any distance, and he does not regard their 

 existence as confirmed. The extremely superficial situation of this body, which 

 makes it resemble a mere thickness of the walls, is paralleled by that of the great 

 ventral nerve-mass in Sayitta. 



The contractile chord which runs to the water-sac is probably attached to a 

 capsular covering of the ganglion, rather than directly to the ganglion itself. This 

 chord, though showing strong contractions by wliich the summit of the larva is 

 drawn down towards the water-sac, is of a homogeneous structure, presenting no 

 appearance of distinct fibrillin or of other contractile elements. 



The author instituted a comparison between Tornaria and M'ttraria. We have 

 ' in both the external transparent pyramidal or dome-shaped body, with a lateral 

 oral orifice and a basal anal orifice, enclosing an alimentary canal which is divisible 

 into three regions, and takes a partly horizontal and partly A'ertical direction in its 

 course from one orifice to the otlier*; we have in both, near the base of the body, 

 the circular band which carries loiig vibratile cilia, accompanied by a row of pig- 

 ment spots, and in both the cushion-like ganglion-carrying ocelli. 



From Mitraria, Tornaria chiefly differs in the presence of the thick sinuous and 

 convoluted bands which give it so close a resemblance to certain Echinoderm 

 larvre, and which are entirely .absent from Mitrnria, and in its water-vascular 

 system, with the contractile cliord which extends from this to the apical ganglion. 

 If a water-vascular system is present in Mitraria, it consists thei-e of a system of 

 sinuses excavated in the walls of the dome, but without any representative of the 

 great central sac. In Mitraria the great apical ganglion carries not only the two 

 ocelli, but also two capsules, probably auditory; these capsules do not exist in 

 Tornaria. In Mitraria the two nerve-chords which the apical ganglion sends down 

 one on each side of the alimentary canal are very distinct ; in Tornaria, if they 

 exist at all, thej^ are by no means obvious. Finally, the ciliary circlet is simple in 

 Tornaria, wliile in Mitraria it is double. 



According to Alexander Agassiz's account of the development of Tortiaria into 

 Bulanor/lossiis, the great transverse circlet of cilia becomes, by the elongation of the 

 body, gradually pushed baclcwards, so as to form the anal ciliated ring of the young 

 worm. In Mitraria the great ciliary circlet remains unchanged in position, and is 

 pi'obably ultimately absorbed, the worm during its development acquiring a new anal 

 wreath of cilia, 



3. Ametrangia hemisjihcerica (uov. gen. et sp.). 



Among the most abundant products of the towing-net was a little hydroid 

 Medusa, remark.able for the want of symmetry in the distribution of its gastro- 

 vascular canals. It is of a hemispherical form, with the base about half an inch in 

 diameter, and provided with very numerous (more than 100) marginal tentacles, 

 which are very extensile, and may at one time be seen floating to a length of three 

 or four inches, and at another coiled into a close spiral against the margin of the 

 Umbrella. Each tentacle originates in a bulbous base with a distinct ocellus. No 

 lithocysts are visible on the margin. The velum is of moderate width. 



The maiiftTjriuni forms a small projection from the suunnit of the umbrella, and 

 terminates in four rather indistinct lips. From the base of the manubrium three 

 rather wide offsets are sent off at equal intervals into the walls of the umbrella. 

 These gradually contract in diameter, and then, as three narrow tubes of uniform 

 diameter, run towards the margin, where they open into the circular canal. The 

 Symmetry of the radiating canals is confined to these three primary trunks. From 

 their wide proximal ends each sends oft" branches, some of which may be traced to 

 the margin, where, lilce tlie three primary canals, they enter the circular canal ; while 

 others can be followed for various distances in the umbrella-walls, in which they 

 terminate by blind extremities without ever reaching the margin. These branches 

 are very irregular in the number sent off from each primary canal as well as in 

 their length and directions. 



* In the species of Mitraria described by J. Miiller and by MecznikofT, both oral and 

 anal orifices ai-p basal, and tlie alimcntjiTy canal presents n U-shaped curvature. 



