673 THE ANIMAL KINGDOM. 



retain the prey. Macrorhynchus helgolandiciis has a poison dart in con- 

 nection with the atrium genitale. 



The form of the spermatozoon is very variable, especially in Rhabdo- 

 coelida. There is a reciprocal copulation, but self-fertilisation occurs in 

 the case of the summer ova of many Rhabdocoela. The ovum is con- 

 tained in a shell, or a number of ova within a common shell or cocoon, 

 as in Tricladida, some Rhabdocoelida, and some Polycladida. The last- 

 named usually lay their ova in rows united by a cement. In some Rhab- 

 docoela (certain fresh-water Mesostomidae] there are thin-shelled summer 

 and thick-shelled winter ova. The egg shell, or cocoon, is formed either 

 by the atrium genitale, by the uterus, or by special glands. In those forms 

 where there are vitellaria, it contains vitelline-cells in addition to the germ- 

 cell. The vitelline-cells retain their integrity, and are devoured by the 

 young Turbellarian at a certain stage of growth. Segmentation is total 

 but unequal, and there is a gastrula formed either by invagination or over- 

 growth. Development takes place within the parent in the case of summer 

 ova (supra). There is a metamorphosis in many Polycladida^ the larva 

 having a praeoral ciliated ring, which is produced first into four ( = Gotte's 

 larva) then into eight ciliated processes ( = Miiller's larva), subsequently 

 absorbed. A few viviparous species are known 1 . 



Asexual reproduction by posterior gemmation occurs in the Rhabdo- 

 coele family Microstomidae ; chains of sixteen individuals may be thus 

 formed in Microstoma lineare. Transverse fission has been observed in 

 the Triclads Planaria subtentaculata, Planaria (sp. ?), and Polycoelis cormita. 



The Turbellaria are carnivorous. They inhabit fresh brackish and salt 

 water, and damp earth. A few Rhabdocoeles are parasitic, the genera 

 Graffilla and Anoplodittm entirely so, the former in marine Mollusca, the 

 latter in or on Holothurians ; so too the Triclad Bdelloura on Limulus. 



Two curious Polyclads have recently been described with certain Ctenophore- 

 like characters. These are the Coeloplana of Kowalewsky (Z. A. iii. 1880, p. 140) 

 and the Ctenoplana of Korotneff (Z. W. Z. xliii. 1886). They resemble Polyclads 

 in the flattened aspect of the body, the general ciliation of the ectoderm, the 

 branched digestive system with ventral mouth. Ctenoplana has an outer longi- 

 tudinal layer of muscles, an inner transverse, as well as dorso-ventral bundles. 

 Both resemble Ctenophora in the presence of two solid retractile tentacles, one on 

 each side, at right angles to the plane of the stomach. There is an aborally placed 

 otolith-mass. In Ctenoplana this lies, supported by the ends of stiff hairs, in an 

 open depression, at the sides of which the nervous system is situated (?), and there 

 are eight short dorsal rows of ctenophoral plates, arranged as in Ctenophora^ and 

 lodged in grooves which can be depressed by special muscles. Two branches arise 



1 See Sillhnan, Z. W. Z. xli. 1885, pp. 59-63. Mesostoma viviparum and Planaria gracilis 

 from the fresh waters ; Vortex Warrenii, marine ; all N. American. 



