ZOOLOGY. 779 



Gastropods. 



The icings or cephalic appendges of Pteropods. — The iiomologies of the 

 wing-like anterior appendages of the Pteropods are still involved in 

 doubt. In the gymuosomatous forms, especially of the genera Clione, 

 Clionopsis, or Pueumodernion, there are always two pairs of tentacles, 

 and Dr. Pelseneer believes that they are ''homologous with the two 

 pairs of the eutliyneurous Gastropods." In thecosmatous Pteropods, 

 there is a pair of tentacles so reduced as to be rudimentary, and if the 

 species do not have eyes when adult, they have them in some stage of 

 development; these tentacles are homologous with the posterior or 

 nuchal oculiferous tentacles of the gymuosomatous Pteropods, while the 

 absence of the anterior is to be explained by the swimming lobes en- 

 circling the head. ''Most of the gymnosomata have a pair of buccal 

 appendages between the two pairs of tentacles, and these, though 

 varied in aspect, are probably similar in origin; it is explained how, in 

 (Jlione, they are really inserted on the external wall of the buccal cavity 

 just as in Cirrifer and Pneumodermon; but at the same time it is to be 

 remembered that this part of the buccal cavity is an introvert and not 

 a true part of the oral cavity." {Quart. Journ. Micr. Soc., v. 25, pp. 

 491-509, 1 pi.; J. B. M. S. (3), v. 6, p. 53.) 



The eyes of Gastropods. — An investigation of the eyes of Gastropods 

 lias been undertaken by Dr. C. Hilger. It appears that there are two 

 kinds of optic organs developed inordinary forms. In the first a rudi- 

 mentary condition is manifested in that the eye "forms but a slight in- 

 vagination of the epithelium of the body ; " this condition is exemplified 

 in the Rhipidoglossate and Docoglossate mollusks, as in the genera 

 Margarita, Trochus, Haliotis, Fissurella, and Patella. In the second, 

 "the eye forms a complete closed capsule, which is invested by connec- 

 tive tissue." This condition is manifested in the Pectinibranchiates, 

 such as the genera Murex, IS^assa, Ficus, Cypraea, and Con us. 



In the former type "the eye has the form of a cup or bell shaped 

 invagination of the epithelium of the body," and "in most cases, the 

 cuticle seems to be developed as a very thin lamella over the anterior 

 part of the vitreous body; the invagination is lined by the retina, which 

 anteriorly passes directly into the epithelium of the body and is in- 

 vested externally by the outspread optic nerve." In the ear-shells 

 (Haliotis) and top-shells (Trochus) "the cavity is filled by a delicate 

 gelatinous substance, and in Patella and Nacella by a finely granulated 

 mass." 



In those forms in which the eyes are more developed "the larger and 

 hinder part of the corpuscle is formed by the retina and the outspread 

 optic nerve, while it is closed anteriorly by the inner cell-layer of 

 the pellucida. Within there is either a lens or vitreous body or both," 

 and "the eye is completely invested by the connective tissue of the ten- 

 tacle," or, as it has been more definitely named, the ommatophor. 



