Characteristics of Lamellihranchiata. xcvii 



covers ; but in some cases, as Pectoi, SpoMlylus^ Trigonia, owing to 

 the absence of the antero-posterior elements of the lattice-work, the 

 g-ills may be reduced to rows of comb-like processes, as in a Pectini- 

 branchiate Gasteropod, or in an osseous Fish. Ordinarily, there are 

 two g-ills on each side; there may, however, be only one, as in 

 Liieina and Corhis ; and in these cases it is always the external 

 pair which is absent. It is later to be developed, and very often 

 smaller, when present, than the inner gill ; and in some cases it 

 serves as a marsupial pouch, in which the ova are impregnated by 

 the spermatozoa brought to them by the inhaled water, and go 

 through certain stages of embryonic development (see p. 6^, infra). 

 The renal organ is always bilaterally symmetrical ; it consists ordi- 

 narily of an excretory sac, which opens into the mantle cavity, and 

 of a secretory lamellar and glaudular sac, which opens internally 

 into the pericardial blood-sinus ; but it may consist only of a single 

 sac on either side, which communicates with its fellow, but prob- 

 ably not with the pericardial cavity. The external orifice of the 

 organ of Bojanus may receive the duct of the generative gland of 

 its own side of the body, or that duct may open within the organ, 

 or independently of it, but at a short distance from its external 

 orifice. 



The Lamellihranchiata never possess any stomato-gastric nor sym- 

 pathetic ganglia; but they may have accessory ganglia developed 

 for the innervation of certain of their organs of animal life when 

 these are largely developed, as in the cases of the siphons of some 

 of the siphonate families, and of the sensory organs developed along 

 the free edge of the mantle lobes. 



The Lamellibranchiata are, with a few exceptions, such as Os- 

 trea and Cyclas, dioecious. The generative glands are always 

 bilaterally symmetrical, and never possess any accessory glandular 

 or intromittent organs. There is no sexual congress in this class; 

 the spermatozoa find their way to the ova either in the cir- 

 cumambient water, or in the cavity of the mantle "■ ; or in that of 

 the outer gill ; or in the cloacal space ; or in the few viviparous 

 species, KeUia, Galeomma, Montacuta, within the ovary itself. The 

 embryos always go through a more or less complex metamorphosis, 



a See Description of Preparations, pp. 54-66, infra ; and Description of Plate V. 

 pp. 193-198, ihique citata. 



9 



