THE DEVELOPMENT OF THE SQUID. 



377 



At the stage shown in Fig. 200, the siphon has sub- 

 stantially its adult form, and is made up of two lateral 

 chambers (si') , which have been formed from the lateral 

 siphon folds, and which open into 

 the mantle-chamber, but have no 

 external openings ; and a single 

 median chamber ('), on the poste- 

 rior surface of the body, which has 

 been formed by the union of the 

 two inner siphon folds, and which 

 opens into the mantle-chamber as 

 well as externally. 



At the point where the lateral 

 chambers meet the median cham- 

 ber, the wall of the siphon is united 

 to the wall of the body, and the 

 three chambers are thus shut off 

 from communication with each 

 other. 



FIG. 201. A free swimming squid, with 

 the external yolk almost absorbed. (Drawn 



from nature by W. K. Brooks. ) 



x* i ( J . - ( ' i . 

 Thj letters as in the preceding figures. 



The animal is so perfectly transparent that the valve- 

 like action of the two outer chambers can be perfectly 

 seen, as their free inner edges are thrown out against the 

 mantle so as to close it at each contraction, and the water, 

 which passes in around the whole free edge of the mantle, 

 is thus concentrated in the funnel-shaped middle chamber 

 of the siphon. 



At about this time the valve of the siphon (Fig. 200, v), 

 is developed as a single unpaired flap, which arises from 

 the posterior surf:iee of the nerk. 



