PTEROPODA AND GASTROPODA. 565 



termination of the oviduct. The secretion in some species is soft, 

 flexible, and transparent ; in most it hardens by contact with the sea- 

 water, and assumes various definite and characteristic forms : the 

 nidus is sometimes simple, sometimes compound, but each compart- 

 ment contains many ova ; and the development of the embryo pro- 

 ceeds in the nidamental chamber until its own little defensive shell is 

 acquired.* 



In the terrestrial Gastropods the ova are usually spherical and 

 opake, and separately extruded. Snails and slugs oviposit in the 

 earth. The tropical Bulmi^ cement leaves of trees together to form 

 an artificial nest for their large eggs. 



The ova of the sea-slug ( Tritonia) are expelled together in the 

 form of a long thread, and are arranged in a spiral manner in the 

 tenacious transparent covering of the thread. In the Doris muricata 

 the ova are aggregated in a flattened spirally disposed albuminous 

 band when excluded from the oviduct. The harder albuminous cap- 

 sules which defend the ova of other marine Gastropods ofier a great 

 variety of forms, some of which are remarkable for their complexity, 

 others for their symmetry and beauty. Here are displayed the nida- 

 mental sacs of the frail Jantliina\\ they are of a flattened pyriform 

 shape, composed of a delicate reticulate film of albumen, and are at- 

 tached by one extremity to a float, formed likewise by a secretion of 

 albuminous matter, dilated into a group of cells filled with air. To 

 this float the parent Jatithina commits her little progeny, and having 

 securely fastened their several cradles or nursery cells, she detaches 

 the float, which bears the ova to the surface, and sustains them where 

 they may best receive the full influence of solar light and heat. The 

 nidamental capsules of the Pyriila rapa^ are attached in regular 

 linear series to portions of decayed wood ; they are of a flattened 

 sub-conical figure, adhere by their apex, and have their base emar- 

 ginate. The nidamental capsules of the whelk || are common objects 

 on our sea-shore ; they are aggregated in large irregular masses, 

 often attached to portions of oyster-shell ; each capsule presents a 

 depressed ovoid figure, with one side convex, the other flat or concave. 

 The small nidamental cells of the Cowry (^Cypred) are aggregated 

 in a flattened group. In the Tmhinella the cells are of a flattened 

 sub-pentagonal form, and adhere together, superimposed one upon 

 the other, forming what is termed a camerated nidus. Each chamber 

 contains between twenty and thirty embryos. In the preparation 

 No. 2950, it may be observed that the rudimental shell is completely 



* CCCLXV. p. 84. pi. vi. t Prep. 2943. B. 



X See Preps. 2945, 2946. § See Prep. 2947. A. || Preps. 2948. 2949. 



o o 3 



