THE GASTROPODA 131 



eggs in the water it surrounds them with a gelatinous mass, like 

 the basommatophorous Pulmonates. 



In the ovoviviparous Gastropoda the progeny are born living 

 after undergoing their development within the parent. They 

 develop in the terminal portion of the oviduct in the following 

 Streptoneura : Paludina, Typhobia; various species of Melania 

 (Fig. 109), Littorina, Cymba, Janthina, Nassopsis, and the Entocon- 

 chidae ; in the Opisthobranch Halopsyche ; and in numerous Pul- 

 monates viz. Glandira algira, Rhytida aequalis, Selenites voyanus, 

 Helix rupestris, H. inversicolor, II. inaequalis, H. unidentata, H. 

 erronea, H. studeriana, Patula cooperi, Acanthinula harpa, the genera 

 Partula, Balea, Coeliaxis, Pupa muscorum, P. umbilicata, P. cylindracea, 

 Clausilia ventricosa, C. similis, Achatina alabaster, and A. zebra, 

 Stenogyra mammillata (Fig. 8), S. octona, S. terebr aster, S. domini- 

 ciensis, S. decollata, S. lamellata, Ferussacia folliculus, F. lamellifera f 



Fio. 109. 



Melania episcopalis, out of its shell, showing the female genital apparatus, right-side view, 

 a, anus ; 6.0, brood-pouch opening ; b.p, brood-pouch ; /, foot ; g.gr, genital ciliated groove ; 

 g.o, genital orifice ; m, mouth ; oc, eye ; od, oviduct ; op, operculum ; ov, ovary. (After Moore.)- 



F. procerula, F. debilis, F'aginula vivipara. The embryos develop in 

 a special incubatory pouch excavated in the interior of the foot 

 and connected with the extremity of the ciliated canal which passes 

 from the female orifice to the head in Melania episcopalis (Fig. 109, 

 b.p.) and Tanganyicia rufofilosa (Fig. 78, b.p). 



In viviparous Gastropods the ovum contains but little yolk,, 

 but in other forms the quantity of deutoplasm is greater, and is 

 especially large in a number of Rachiglossa, such as Nassa, etc. 

 The segmentation of the ovum is always total, and, except in cases- 

 in which the deutoplasm is scanty (Paludina), it soon becomes 

 irregular. As a rule in Aspidobranchia, Taenioglossa, and Pul- 

 monata, the two first cleavage planes are meridional, the first 

 separating the right and left halves of the future animal. The 

 third cleavage is equatorial and cuts off the micromeres at the 

 animal pole from the macromeres at the opposite pole, so that, as 

 a rule, there are four macromeres from the beginning (Fig. 9, A, B). 

 These macromeres give rise to two more generations of micro- or 



