504 



EVOLUTION OF ANIMALS 



Part V 



Digestive 

 cavity 



Nerve net 



Circular 

 muscle 



Longitudinal 

 nnuscle 



Mesenchyme 

 Excretory pore 



Epidermis 



sa?jflfissJs; 



»^^WSS]^S3ffi! 



Rhabdites 



Dorso- ventral 

 muscles 



Nerves and 

 nerve net 



Ventral 

 nerve cord 



Fig. 25.5. Cross section of a mature planarian. A net-like tissue of the mesen- 

 chyme occupies the space that in higher animals is taken by the body cavity. The 

 excretory organs are not shown. The glandular rhabdite cells form and discharge 

 minute bodies, the rhabdites, largely composed of calcium. 



temperature, to water currents, to currents of electricity, and to gravity. 



Reproduction. Most planarians are hermaphrodites having complete male 

 and female systems in the same individual. In spite of this, they mate and the 

 sperm cells of one are placed in the female passages of the other and vice 

 versa. 



Both male and female systems are complex (Fig. 25.6). The male sys- 

 tem consists of hundreds of minute testes, each connected by a micro- 

 scopic tube that joins a larger tube (vas deferens) , one on each side of the body. 

 These connect with the median seminal vesicle which serves as a storage for 

 the sperm cells before they are released at mating. The eggs are fertilized as 

 they are discharged from the ovary. Yolk cells, from the yolk glands, adhere 

 to the outside of the fertilized egg and in this unique way supply it with food. 

 As in a hen's egg, yolk is universally inside. As the planarian's one or more 

 eggs with their yolk cells are moved along the oviduct, the latter secretes a 

 capsule about them. Such capsules are commonly fastened to the undersides 

 of submerged rocks; those of Dugesia resemble fig seeds on short stems. 

 Capsules collected from rocks usually hatch in two or three weeks if kept in 

 clean, cool water and subdued light at ordinary temperatures. 



Planarians commonly reproduce asexually by transverse division or fission. 

 Fission is most common during the summer, sexual reproduction in winter and 

 spring. When about to divide, the animal suddenly fastens its rear end down 

 and pulls its front end forward, till the two separate. In a lightly greased dish, 



