3 8o 



MOLLUSCS. 



EGG - CAPSULES OF 



Purpura lapillus. . 



Like the nmrices, these molluscs are devourers of Bivalves. The annexed illustration 

 represents the egg-capsules of the common British P. lapillus. The animal and 

 shell of Acanthina are similar to those of Purpura, but the outer lip of the aperture 

 is furnished at the anterior end with a more or less prolonged conical tooth or 

 prong. Rapana, Concholepas, and Sistrum are other nearly- 

 allied forms. The members of the family Coralliophilidce 

 resemble the purpuras, but are characterised by T the want of a 

 radula, and their singular mode of life, dwelling either in or upon 

 corals. Rhizochilus in early life is like the young of Purpura, 

 but subsequently it attaches itself to barked corals {Antipathes), 

 living a prisoner's life, immured within its own shell. It pro- 

 longs the lips of the shell around the stems of the coral, completely closing the 

 aperture, with the exception of the siphonal end, which it extends into a distinct 

 tube, serving to carry both water to the gill and food to the mouth. What the 

 nature of its nourishment may be — possibly derived from the Antipathes itself — 

 and what may be the reason of this self- 

 immurement, are problems to be solved. 

 ( 'orallioph ila madreporarum attaches 

 itself to corals ; other species live a more 

 active life, crawling over the surface of 

 the coral. Several are tinted with lilac, 

 and have purple apertures ; others, however, 

 are entirely white. Leptoconchus and 

 Magilus are both dwellers in coral. The 

 former lives in crypts excavated in brain- 

 stone corals, and resembles, as regards the 

 shell, the young state of the latter. 

 Magilus affects coral-reefs, and is remark- 

 able for the great alteration which takes place in the course of its career. At 

 first it assumes the form of an ordinary spiral shell, and takes up its abode 

 within the crevices of growing coral. The coral slowly advances around the shell, 

 and would soon enclose it, if the mollusc were not provided with some means of 

 defeating this. The creature prolongs the lips of its aperture into a long, but 

 mostly crooked tube, so as to keep pace with the growth of the coral, and keep 

 the tube open for the. free ingress of the water. As the tube increases it becomes 

 too long for the animal ; consequently the mollusc tills up the spiral portion, and 

 as much of the tube as is not required. These shells are found in the Red Sea, 

 and the Indian and Pacific Oceans. 



Rhizochilus antipathum. a, Young ; b, Adult. 



SECTION T.ENIOGLOSSA, 



The majority of the molluscs included in this section of Pectinibranchs are 

 marine forms ; although a few families are found in fresh water, while others are 

 terrestrial and air-breathers. Typically, the radula has seven rows of teeth, one 

 central, and three laterals on each side ; but there are a few families in which this 

 armature is modified. Sometimes there is only a single lateral on each side ; but, 



