THE CGELOMATA INVERTEBRATA 185 



to tail directions in such a way that the opening of the vasa deferentia 

 of the one come to lie opposite to the openings of the spermathecae 

 in the other and vice versa. The sperms are then transferred from 

 each worm to the spermathecae of the other, where they can live 

 for some time until required, after this the worms separate. It is 

 these spermatozoa that fertilise the ova of the worm in which they 

 are lodged, and consequently the ova of one worm are cross-fertilised 

 by the sperms obtained from another. Some time after copulation 

 the clitellum secretes freely a substance which as it hardens forms 

 an elastic band, the cocoon, round the worm and at the same time 

 it also secretes into it a nutrient fluid. The worm then wriggles 

 backwards in this, and when it comes to lie opposite to the oviducal 

 pore the eggs from the receptaculum are laid in it, so that it can also 

 be spoken of as an egg capsule. Further wriggling brings it over 

 the spermathecal pores, and here the sperms are discharged and 

 fertilisation takes place, one sperm entering each ovum. A supply 

 of albuminous fluid is added to the contents of the cocoon to serve 

 as food, and finally the worm completely escapes from it and it 

 automatically closes at its two ends by its own elasticity. The 

 cocoon when left behind in the earth is a toughish oval sac of yellow 

 or yellowish-brown colour about 5 or 6 mm. in length. Although 

 a large number of eggs are laid and even fertilised, only a few or 

 perhaps even one alone undergoes complete development. The 

 nutritive fluid serves as food for the growing embryo, which when it 

 leaves the cocoon is already a minute little worm but quite complete, 

 and it only needs to grow in order to become a mature adult. Thus 

 we have no free living larval stage to correspond with the planula 

 of Hydra, and the development is direct from an egg through an 

 embryo to the definitive adult form. 



As in the frog, the nervous system of Lumbricus- may be 

 divided for convenience of description into a central and a peripheral 

 portion, and so represents a condition considerably in advance of 

 that in Hydra, for it is a concentrated and not a diffuse system. On 

 the internal side of the ventral body wall in the middle line the'nerve 

 cord is to be seen as a long whitish cord exhibiting an enlargement or 

 ganglionic swelling in each segment of the body. In the fourth 

 somite the swellings are quite marked, forming the sub-pharyngeal 

 ganglia, which diverge from one another and are continued forward 

 around the pharynx as two moderately fine nervous strands, the 

 circum-pharyngeal connectives. These pass on to the dorsal side of 

 the alimentary canal and terminate in the third somite in two 

 conspicuous enlargements touching one another in the middle line, 

 the so-called cerebral or supra-pharyngeal ganglia. Although 

 appearing as a single strand the ventral cord is, morphologically 



