TRANSACTIONS OP THE SECTIONS. 107 



one on tlie right and the other on the left side, extending transversely from the 

 dorsal to the ventral surface. 



The ciliated band which runs round the base of the dome possesses a rather 

 complex structure. It consists of two concentric rings — an outer one composed of 

 large, oral, distinctly nucleated cells ; and an inner one of a granular structure and 

 yellowish colour, in which no distinct cells could be demonstrated. The cilia form 

 two concentric wreaths borne by the underside of the band — an outer wreath con- 

 sisting of very long cilia, and borne by the inner edge of the outer portion of the 

 band 5 and an inner wreath of much shorter cilia, borne by the inner edge of the 

 inner portion. The band, with its cilia, is inteiTupted for a very short space at the 

 aboral side of the dome. There is probably at this spot an entrance into a water- 

 vascular system. No such system, however, was observed in the specimen, though 

 the author had described in another species of Mitraria a system of sinuses which 

 appear to exist in the walls of the dome, and which he regarded as representing a 

 water-vascular system (Brit. Assoc. Report for 1872). 



Occupying the very summit of the dome is a large, somewhat quadrilateral 

 ganglion, from which two distinct filaments are sent down, one on each side of the 

 alimentary canal ; but he was not able to follow these filaments to their destina- 

 tion. The bilateral symmetry of the ganglion suggests its formation out of two 

 lateral halves. Though its very superficial position gives it the appearance of 

 being a mere thickening of the walls, the view here taken of its being a nervous 

 ganglion seems to be the only one consistent with its relations to the surrounding 

 parts. 



On each side of the pharynx, a little behind the mouth, is a small oval ganglion- 

 like body, from which a filament runs to the ciliated band. Some delicate filaments 

 may also be seen lying between the pharynx and the walls of the dome on which 

 they seem to be distributed 5 but the author could not trace them to any distinct 

 ganglionic centre. 



The great apical ganglion carries two very obvious black ocelliform spots, and, 

 besides these, two clear vesicles enclosing each a clear spherical corpuscle. The 

 two vesicles may probably be regarded as auditory capsules. 



The further development of this larval form has not been observed. It probably 

 consists chiefly in the continued prolongation of the alimentary canal bej'ond the 

 base of the hemispherical dome, the completion of the annulation by its extension 

 to the right and left sides, and the gradual contraction of the dome and final 

 absorption of the ciliated band. 



2. Toi'naria, 



Two specimens of the larval fonn originally discovered by Johann Miiller, and 

 described by him under the name of Tornariu, were obtained ; but these unfortu- 

 nately perished before a sufficiently exhaustive examination of them could be 

 made. On the whole, their structure agrees closely with what has been pointed 

 out by Alex. Agassiz, in his valuable and elaborate memoir on Tomaria and 

 liahnwglossus. The species appears to be different from those hitherto described. 

 The gills had not begun to show themselves, and there were but traces of the 

 " lappets " described in other species as appended to the posterior extremity of the 

 stomach. 



The author believed that he could distinguish a minute ganglion on each side of 

 the oesophagus ; filaments were sent off from it to the neighbouring parts, and the 

 two were connected to one another by a suboesophageal commissure. The water- 

 vascular chamber was very distinct, but the so-called heart was not observed ; 

 wliile within the body-cavity, lying close to the dorsal pore and over the canal by 

 which the great water-sac communicates with the external medium, was a small, 

 closed, rather thick-walled vesicle, containing numerous oval corpuscles. Of the 

 nature of this vesicle the author could not ofter any opinion. 



The cushion-like body which occupies the summit of the larva, exactly as in 

 Mitraria, and supports the two ocelliform spots, was very distinct ; and so also was 

 tlie contractile cliord which extends from this to the walls of the water-sac. The 

 author, however, could not here, any more than in Mitraria, regard the cusliion-like 

 body as a mere thickening of the walls : he believed it to be a nerve-mass, and 



8* 



