TUNICATA, BRACHIOPODA, 267 



increase in length they cross the middle line of the sternum, and inter- 

 lock with those of the opposite side. The mouth, the vent, and the 

 wide alimentary canal are formed ; the integument is completed, as 

 in other Articulata, by a dorsal cicatrix, and in this state the young 

 spider breaks through the attenuated chorion. The head, the man- 

 dibles, the thorax, and abdomen are first extricated, and afterwards, 

 but with more difficulty, the palpi and legs are withdrawn. It very 

 soon has to repeat a similar process in throwing off its foetal integu- 

 ment, which becomes too small for its rapid growth. This moult 

 always takes place in the silken nest of the parent ; the young spider 

 then issues forth, and is subject to repeated moults before acquiring 

 the mature size. 



We perceive, therefoi-e, that throughout the whole process of the 

 development of a spider, there is nothing worthy to be called a meta- 

 morphosis. The highest of the Articulata indicates in the feeblest 

 manner, and can scarce at any period be recognised in the condition 

 of, the apodal and acephalous worm ; but, with the first rudiments of 

 trophi and legs, the characteristic or special form is acquired. 



The regeneration of the legs of the spider follows precisely the 

 same law as that which regulates their reproduction in the Crustacea. 

 If the limb be injured at the tarsus, tibia, or femur, it must first be 

 cast off at the coxo-femoral joint before the process of reproduction 

 can commence, and this must be preceded by a moulting of the in- 

 tegument ; the new leg being at first of small size, but with all its 

 joints and appendages, and acquiring the full proportions at the 

 second moult. 



LECTURE XX. 



TUNICATA, BRACHIOPODA. 



The Articulate series of animals leads the investigator of the ascend- 

 ing course of organic development from the vermiform Zoophytes to 

 the higher organised worms which circulate red blood, and through 

 the strange and changeable forms of Epizoa and Cirripedia to the 

 Crustacea, the Insecta, and the Arachnida, in which three classes of 

 articulated animals with jointed limbs, as many diverging branches 

 from the common vermiform root, seem respectively to terminate. 



Yet having attained these different summits of the articulate branch 

 of organisation, the enquirer still finds himself at a great distance 



