86 LECTURE V. 



They differ, in fact, chiefly by their ciliated external surface : and 

 they form the order Turbellaria of Ehrenberg. Their soft paren- 

 chyme shows, in many species, immediately under the ciliated integu- 

 ment, peculiar corpuscles resembling the "thread-cells" of the 

 Hydrozoa and Acalephce : some of these cells contain six, eight or 

 more spiculiE. The whole parenchyme is remarkably contractile, 

 and they creep or swim by movements of the whole body, as well as 

 by the action of the vibratile cilia : where muscular fibres are obvious 

 in the larger Planarice, they are smooth. The chief part of the 

 nervous system consists of a pair of ganglions, sometimes confluent, 

 near the head, from which many filaments diverge, the largest and 

 longest pair being continued backwards. More or fewer of the 

 smaller anterior nerves terminate in minute bulbs beneath the co- 

 loured eye-specks, which are clustered in one or more groups on the 

 upper surface of the forepart of the body. Exploratory organs or 

 processes are sent off from the anterior border of the Planaria ten- 

 taculata. 



The mouth is placed either at the anterior end of the body, or 

 beneath that end, or under the middle of the body, and these differ- 

 ences of position serve as generic characters. In certain Planarice 

 the digestive canal is continued from the muscular pharynx to the 

 hinder end of the body, where it terminates in a blind end : this 

 simple type characterises the family Rhabdocisli. In the rest of the 

 order the pharynx can be everted, like a proboscis ; it leads to a 

 wide, more or less elongated, canal, from which numerous branched 

 blind tubes radiate into the surrounding parenchyme : this type of 

 the digestive apparatus characterises the Dendrocceli. In the latter 

 Planarice there are two lateral vascular trunks which anastomose 

 together at both ends of the body. In the Rhabdocceli there are one 

 or two pairs of vessels, which form loops at the extremities of the 

 body, and do not send out branches. These vessels contain a clear 

 colourless fluid in both families, and have ciliated lobes or surfaces 

 at certain parts. 



The male and female organs are so combined in the same individual 

 in the larger Planarice, that self-impregnation may be possible ; but 

 reciprocal impregnation seems to be the rule : and the twofold 

 connection of two hermaphroditical individuals has been witnessed 

 by Von Baer, Duges, and other observers of these non-parasitic 

 Sterelmintha. The two ovaria are diffusedly branched through the 

 parenchyme of the body, and terminate in a capacious oviduct or 

 "bursa copulatrix." A double testis transmits the sperm with 

 moving capillary spermatozoa through two convoluted sperm-ducts 

 into a sperm-sac, with which an erectile organ is connected. A 

 common generative outlet, close behind the mouth, serves for both 



