PTEROPODA : GASTROPODA. 469 



duct opens near to but in front of the anus. In Pneumodermon a copulatory 

 organ lies within the aperture of the duct, but in other Pteropoda it lies at 

 some distance in front of the aperture. It is always in- and e-vaginable 1 . 



The ova are laid in a mass of albumen, and are usually found floating 

 on the surface of the sea. Clio attaches the mass to sea-weeds when confined 

 in aquaria. Segmentation is unequal. The velum is bilobed and large, 

 except in Hyaleidae. The larva of Gymnosomata (Clio, Pneumodermon) 

 loses the velum and shell, and then passes into a second larval form pro- 

 vided with three zones of cilia girding the body one anteriorly, in front of 

 the foot ; the other near the middle ; the third posteriorly. The first 

 atrophies early ; the middle one has been observed in a nearly adult Clio, 

 and the hindmost in a Pneumodermon. 



The Pteropoda are exclusively pelagic and carnivorous. They are 

 found in all seas, sometimes, e.g. Clio, in immense numbers. Fossil forms, 

 all extinct, appear in Cambrian strata. 



There are two orders : 



(1) Thecosomata, with mantle fold and shell; with three living families, 

 Hyaleidae, Cymbuliidae, and Limacinidae (Spirialis\ and the fossil families Conula- 

 riidae, Tentaculitidae, and Thecidae. 



(2) Gymnosomata. Mantle fold and shell absent in adult; a second larval form 

 with three ciliated bands ; with the living families Clionidae, e. g. Clio (or Clione) 

 borealis, Pneumodennonidae, and Pterocymodoceidae. 



Keferstein, Bronn's Klass. und Ordn. des Thierreichs, iii. 2. Clio borealis. 

 Wagner, 'Die Wirbellosen des Weissen Meeres,' Leipzig, 1885. Cymbulia, external 

 shape and position in shell, Macdonald, P. R. S. xxxviii. 1884-5. On certain 

 Gymnosomata, Boas, Z. A. viii. 1885. 



Cephalic appendages of Gymnosomata, especially of Clio, Pelseneer, Q. J. M. 

 xxv. 1885. Structure of suckers, Niemiec, Recueil Zool. Suisse, ii. 1885. 



Histology of Pteropoda, &c., Paneth, A. M. A. xxiv. 1885. 



CLASS GASTROPODA. 



Glossophora, with a foot, which, except in certain swimming forms, is 

 simple, median, and flattened into a broad, sole-like surface by the con- 

 tractions of which the animal crawls. It is often divided into three succes- 

 sive regions, pro-, meso-, and meta-podium, by lateral constrictions. 



1 Wagner (op. cit.) states that Clio possesses, (i) a short copulatory organ containing a male 

 receptaculum seminis ; (2) a long excitatory organ; both of them in- and e-vaginable. When two 

 individuals meet, they perforate reciprocally each other's body, with the excitatory organ, from 

 which masses of granules are discharged into the coelome. They separate, after having mutually 

 filled the male receptacula. Congress of each of these two individuals with an ovigerous individual 

 is necessary for the fertilisation of the ova, the sperm received on the first occasion being now trans- 

 ferred. 



