ART. 5. — N. YATSU I 



I. INTRODUCTORY. 



Regarding the histology of Lingula there have hitherto 

 appeared only two articles. One of them by Beyer ('86) is 

 very far from being satisfactory, considered from the present state 

 of onr science. The other is the beautiful work of Blochmann 

 ('oo), thanks to which the main organization and some of the 

 finer structures of Lingula have been made thoroughly clear. 

 It must however be confessed that a greater part of the histology 

 of this form still remains in the dark. The present piece of 

 work, which has been undertaken in connection w T ith my em- 

 bryological studies, is thought, fragmentary though it be, to be 

 worth publishing, as it may help to throw some new light upon 

 the subject in question. As for other points I shall reserve them 

 for a future occasion. 



II. FORMED ELEMENTS OF THE CŒLOMIC FLUID. 



In the cœlomic fluid of Lingula, which fills up the body 

 cavity, the palliai sinus and the peduncular cavity, and bathes 

 all the viscera and muscles, we find three kinds of formed 

 elements : viz. 1) blood corpuscles, 2) leucocytes, and 3) spindle 

 bodies. Besides these there occur encysted sporozoa, and very 

 rarely a Distomum, closely allied to those found in Balanoglossus, 

 Amphioxus, and Gephyrea. 



1. Blood Corpuscles. 



The blood corpuscle (PI. I., Fig. 1) exceeds in number all 

 the other elements. It is colorless, transparent and somewhat 



