NOTES AND MEMORANDA. 3 iS 



pits of an embyro Vertebrate to the closed vesicles into which 

 they become converted. 



In the Trachymedusee the auditory organ appears more in the 

 form of a modified tentacle, and consists, in its simplest condition, 

 of a papilla, formed of a central axis of endoderm and a coatino- 

 of ectoderm. In the terminal cells of the endodermal axis is 

 situated a concretion, and some of the ectodermal cells are morlified 

 so as to form hair cells. In the more complicated types the 

 whole papilla becomes enclosed in a cup, and in the highest 

 forms {e.g. Geryonia) by the conversion of the cup into a vesicle, 

 the papilla comes to be situated in a completely closed cavity. 

 In Geryonia the entrance of a nerve into the vesicle, originally 

 described by Hfeckel, is fully established by the brothers Flertwig 

 and by Eimer, and the termination of its fibres in the hair cells 

 around the endodermal axis is clearly demonstrated by the former. 



An optic organ is confined to the Ocellata. In its lowest 

 conditions it consists of certain areas at the base of the tentacles 

 composed of sense-cells invested by pigment-cells, and in its 

 most differentiated condition a lens formed by a thickening of 

 the external cuticle is added to the structures found in the sim- 

 pler form of eye. 



The sense organs of the Acraspeda have been investigated 

 both by Schiifer, the brothers Hertwig and by Glaus. The 

 brothers Hertwig have studied with great detail almost all the 

 main types of the Acraspeda and the observations of Claus have 

 also been made on more than one form, while those of Scliiifer 

 are confined to Aurelia. The general type of sense organ 

 appears to be auditory in function and is more or less similar to 

 that of the Trachymedusse. It consists of a tentacle-like organ 

 situated in a groove on the under surface of one of the lobes of 

 the edge of the disc. The groove is nearly converted into a 

 canal by the presence of a fold or fl;ip closing it over below. 

 The organ itself is somewhat knobbed and consists of an endo- 

 dermal axis along half the length of which is continued a pro- 

 longation of the gastrovascular canal- system, while its terminal 

 portion is solid and contains calcareous concretions. The ecto- 

 derm covering the knobbed extremity is flat but around the base 

 its cells become columnar and are provided each with a stiff 

 hair or bristle and prolonged internally into a nerve fibre. Thev 

 constitute, in fact, what have already been described as the ganglia 

 of the central nervous system. Between the ectodern and entodern 

 is a structureless lamina, which is spoken of as mesoblast by 

 Schiifer though it appears to us hardly to deserve that title. 

 Optic organs of a simdar character to those in the Ocellata are 

 added to the sense organs in Nausithoe, Aurelia, and Charybdea. 



VOL. XVllI. Z 



