ON THE DEVELOPMENT OF LINGULA ANATINA. 39 



the arm-apparatus swells out dorsally, as is seen in Fig. 78 

 (PL V.). 



In the oesophagus is a valve-like structure which reduces its 

 calibre (œs.) (PL V., Fig. 75.). The stomach has differentiated 

 into two parts : the so-called liver, and the mid-gut. The former 

 occupies the main part of the stomach and is fdled with a number 

 of unicellular algae which are taken into the liver cells as food. 

 The mid-gut is represented at this stage as a small ciliated area. 

 The intestine has already made its appearance. 



X. STAGE OF 5-6, AND 7-9 PAIRS OF CIRRI. 



No account of the foregoing stages has hitherto been published, 

 but the later larvae, as we noted, have been studied in detail. 

 The work of Brooks ('78) which is based upon materials whose 

 earliest stage was the larvae of 5 p. c. stage (the sixth pair was 

 about to be formed), leaves little to be desired as far as a know- 

 ledge of surface changes and the outlines of organology is con- 

 cerned. As sections, however, were not employed in the work of 

 the American author, 1 there are, of course, many details lacking 

 in the history of the later development, and in my present 

 account I shall endeavor to describe some of the detailed changes 

 not yet touched upon by any previous writer. 



The 5-6 p. c. stage is of a special interest, since it is at this 

 stage that the Anlagen of many important organs are laid. It 

 would, of course, be desirable to give a detailed account of these 

 larvae, and afterward of the next stage, but, as there are no 

 sudden changes between the 0-6 p. c, and 7-9 p. c. stages, I 



1. Brooks ('87) promised that lie would describe sections of Linfjula-lnvvœ. in other 

 places (p. 90), but he, so far as I know, has not fulfilled his promise. 



