ANATOMY AND PHYSIOLOGY OF A SNAIL. 



43 



elongated or ovoidal mass (nidamental mass), which is fastened to 

 a stem or a leaf of an aquatic plant, or often to stones and 

 bricks that lie on the bed of the pond or river. The Bulini 

 make a nest for their eggs by cementing together leaves of 

 trees. Paludina vivipara and P. contecta are ovoviviparous among 

 water-snails ; Helix rupestris^ a few species of Clausilia and Pupa, 

 a Vitrina and an Achatinella are viviparous among land-forms. 



The different processes which the egg undergoes in its develop- 

 ment cannot be described here in detail, because they are of too 

 complicated a character to be well understood by the beginner. 

 Enough, however, will be stated to render the reader familiar 

 with the more striking changes, and should he wish to advance 



Fig. 12. Some stages in the embryonic development of Planorbis (after C. Rabl). 

 a, optical section through a segmenting ovum (24 segments). Rk, polar bodies ; Fk, 

 segmentation cavity, b, stage with four mesoderm cells viewed from the vegetative 

 (lower) pole ; Ms, mesoderm cells ; En t endoderm ; EC, ectoderm, c, oblique optical 

 longitudinal section through the stage with four mesoderm cells, d, older embryo in 

 which the shell-gland has shifted to the right, Sdr, shell-gland ; s, shell ; O, mouth ; 

 D, alimentary canal ; R, commencing radula ; Sp, apical plate (thickening of praeoral 

 lobe) ; Oc, eyes ; ot, otolith ; N, primitive kidhey ; Ve, velum. 



farther into the subject he may consult the embryologicil 

 text books in general use, such as Balfour's " Comparative 

 Embryology," and Haddon's " Introduction to the Study of 

 Embryology," or the article " Mollusca " by Professor Ray 

 Lankester in the sixteenth volume of the last (ninth) edition of 

 the Encyclopedia Britannica. Taking Limncza stagnalis as 

 a typical instance, the ovum first divides into two and then 

 into four portions, each division being preceded by the division 



