702 Chronicles of Science. [Oct., 



with one moiety of the mantle of the bivalve, while others regard 

 the operculum as homologous with the byssus. Dr. Morch reviews 

 these opinions in the ' Annals,' and traces the cleft of the mantle in 

 some univalves, as in the dentated furrow of Monoceros, Ancillaria, 

 Pseudoliva, and chiefly in Carinaria, in which genus the keel is 

 formed by the two sides of the shell, which are pressed against each 

 other in such a manner that a piece of paper can be introduced into 

 the middle of the keel as far as the foetal shell. The shell of the 

 young Dentalium is also split throughout its whole length. Also in 

 the Acephala nearly all the organs are double ; there are two ovaria 

 with distinct orifices, two kidneys, two pairs of labial palpi, two pairs 

 of gills. It seems therefore probable that the Acephala have also 

 two shells originating in the same way. This duplicity is very 

 indistinct in the univalves, and becomes rarer and rarer in the higher 

 mollusca. The larva of Anodonta has in each shell a distinct byssus 

 bundle, and a distinct intestinal channel with distinct oral orifices. 

 Does the opercular lobe then, with its operculum, represent one lobe 

 of the mantle and its shell in the bivalves, or is it something else ? 

 The fact of the operculum co-existing with the byssus in the same 

 univalve, which sometimes happens, proves that Loven's opinion that 

 the operculum is homologous with the byssus cannot be correct. It 

 must be also remembered that it is not known how the byssus of 

 univalves is formed. The float of Ianthina which attaches the 

 animal to the surface of the water is probably homologous with the 

 byssus, judging from its ventral position. In Acephala it is generally 

 corneous, but in Anomice it forms a calcareous plate (the plug), 

 possibly corresponding with the opercular valve in Hipponyx, which 

 may be considered as a calcareous secretion of the ventral face of the 

 foot. The epiphragm of the Helices would also be homologous, if 

 this plate be really a secretion of the foot, but it is probably secreted 

 by the mantle like the septa of Vermeti. It appears that all parts of 

 the skin in Mollusca can secrete a shell. In the Bullidfe and some 

 Pellibranchs there are thick calcareous plates in the stomach. 



As supplementary to the remarks lately made upon the special 

 hairs of Crustacea, it may be added that MM. Claus and Sars have 

 independently investigated the Schizopods of the family Euphausidae 

 with regard to the organs alluded to by Dana, Semper, and Kroyer, 

 and regarded by Semper as eyes and by Kroyer as auditory organs. 

 They are spherical bodies of a reddish colour, situated at the base of 

 several of the thoracic legs and at the first four pairs of abdominal 

 appendages. Both of the aiithors above mentioned have demonstrated 

 the correctness of Semper's view, although besides these pedal eyes, 

 the animals possess the two large compound eyes common to all 

 Decapods. Each of the thoracic and abdominal eyes receives a special 

 nerve from the ventral ganglionic chain. The organ itself is a 

 spherical bulb moved by special muscles, and in it may be distinguished 

 a crystalline lens, a vitreous body, a pigment layer, and a retina of 

 complex structure. The existence of a crystalline lens distinct from 

 the cornea is very striking, for in other Crustacea no true crystalline 

 lens exists, its function being performed by the thickened and inflated 



