METRIDIUM MARGINATUM MILNE-EDWARDS 51 



column they form a real sphincter columnce. The muscles of the 

 primary mesenteries, all endodermal, are arranged in a definite 

 manner. On one surface of the mesentery is the longitudinal 

 muscle banner and a radial basal muscle. On the other sur- 

 face are transverse muscle fibres, oblique muscle fibres, and a 

 basal radial muscle. 



Nervous system. The nervous system consists of a 

 subepithelial ectodermal and a subepithelial endodermal net- 

 work of gangl ionic cells and fibres. The network is especially 

 rich in the gullet, peristome and tentacles. It is very probable 

 that there is a direct connection between the two networks in 

 the shape of fibres which pass through the mesogloea. Sense 

 organs are absent but sensory cells with tactile and chemo- 

 tactile functions are found both in the ecto- and endoderm. 



Reproductive system. The reproductive organs have 

 been already described. They are in the shape of ductless gonads 

 situated in the mesenteries. The sexual cells are dehisced into 

 the exocceles. The sexes are separate. The ciliated planula 

 develops into a polyp. 



Asexual reproduction. Reproduction by budding from 

 the base of the column near the foot is not uncommon. Occasion- 

 ally a longitudinal fission of adult specimens also occurs. If the 

 fission is not completed the result is a specimen with two mouths 

 and gullets. 



Instructions 



1. Examine a specimen in a deep finger bowl filled with 

 water. Make a life-size drawing showing the side view. Label 

 foot, column or scapus, capitulum, and tentacles. 



2. Make a life-size drawing of Metridium showing the capit- 

 ulum from above. Label tentacles, peristome, mouth and si- 

 phonoglyphs. Label the type to which the specimen belongs, as 

 monoglyphic or diglyphic, depending upon the presence of one or 

 two siphonoglyphs. 



^. Take the specimen in your left hand, press it gently with 



