214 ANATOMY OF SPECIAL FORMS. 



traced (figs. 4, 6). Though I found the refracting lens-like body occasionally very conspicuous, it 

 was frequently impossible to discover it, and it seems to be very soft and easily broken down in the 

 mass of the pigment. This will probably explain the fact that while it was detected by Quatrefages, 

 Krohn, and Claparede in the Eleutheria- and Clavafella-medusai described by them, it eluded the 

 attempts of Hincks to discover it. The correctness of Quatrefages' statement of the evidence of 

 a cornea in the ocellus of Eleutheria is disputed by Krohn ; my own observation, however, of a 

 convex extension of the transparent ectoderm over the surface of the ocellus in Clavatella tends, 

 on the other hand, to confirm it. 



Just above the base of the tentacles the medusa is encircled by a zone of thread-cells 

 imbedded in the ectoderm. 



I have never met with the male, and, with the exception of Krohn, who saw it once, no 

 other observer seems to have been more fortunate. I found Clavatella jjroUfera, in autumn, on 

 the west coast of Scotland, with its medusas carrying eggs (fig. 8). These were contained 

 between the endoderm and ectoderm of the dorsal or proximal side, as had been already noticed 

 by Quatrefages in Eleutheria, and by Hincks and Krohn in Clavatella. They were few in 

 number, not more than five or six, were of comparatively large size, and by their pressure from 

 within gave rise to prominent bosses on the dorsal surface of the medusa. In every case in 

 which I noticed them they seemed to have passed the earlier stages of their development, for 

 their germinal vesicle was no longer visible ; but I lost my specimens before tracing the ova, as 

 Krohn had done, into the condition of "embryos," which he informs us they attained while still 

 in the dorsal brood-chamber of the medusa. From this chamber it would seem, according to 

 the observations of Krohn, that they ultimately escape by the rupture of the ectoderm which con- 

 fines them, and then swim away in the form of free ciliated planulse. 



Claparede' and Filippi^ have also described the gonophore of a Clavatella, but their accounts 

 differ from that just given in some important points. In Claparede's medusa, which seems distinct 

 from that of the Clavatella prolifera, the ova lay beneath the ectoderm of the distal or lower side, 

 whence they projected externally. Filippi, though finding the ova in a dorsal chamber, will not 

 allow that this chamber is formed by the separation of the ectoderm and endoderm, as in all other 

 Hydroida. He regards it, on the contrary, as a special cavity, bounded on all sides by the endo- 

 derm — a view which is almost certain to have originated in some error of observation. 



The medusa makes its appearance as a minute, closed, pyriform sac (fig. 1), consisting of a 

 layer of ectoderm lined by a layer of endoderm, and having its cavity in communication through 

 its peduncle with that of the hydranth. As development proceeds, its cavity may be seen to 

 extend itself at the distal side in six short, tubular, radially disposed offsets, lined by the endo- 

 derm, and prolonging themselves into the substance of the thickened ectoderm of the distal end 

 of the bud. These are the incipient radiating canals, and it is certain that about this time each 

 becomes united to its neighbour by a transverse communication, the commencement of the cir- 

 cular canal, though the actual formation of this communication has escaped me. 



Up to this point tiie ectoderm is continued evenly over the whole bud ; but it now begins to 

 show, at the distal side of the bud, six little elevations, which correspond to the distal extremities 

 of the radiating canals (fig. 9). These elevations gradually increase in length, while an exten- 



' Claparede, ' Beobachtungen iiber Anat. und Entwicklungesch. wirbellos. Tliiere,' p. 4. 

 ' Filippi, ' Mem. della lleale Academia d. Scieuze di Torino,' Ser. 3, Tom. xxiii. 



