ON THE DEVELOPMENT OF LINGULA ANATINA. 87 



At tins stage with the change in the mode of life from the 

 free- swimming to the sedentary condition, the setœ undergo a 

 striking change. Not only a rapid increase in length but also 

 an unequal growth takes place among them. The longest setee 

 measure 250-300 /* in length and 2-3 y- in diameter, and are 

 planted along the anterior margin and two latero-posterior corners 

 of the mantle : in other portions the setae are of a smaller size 

 (PI. VI., Figs. 86, 87). In a general way it may be said that 

 the difference in the length of these structures is correlated with 

 the sedentary life in the mud : the circulation of water in the 

 mantle cavity commences, and, at the entrance of the water 

 current, the set« subserve a function similar to that of our eye- 

 lashes. This difference in length of the setae becomes more pro- 

 nounced in the adult. On a careful examination we may at once 

 convince ourselves that the seta shows longitudinal striation and 

 regular segmentations, each segment measuring 8-10,« (PL VIII., 

 Fig. 132). 



Palliai sinus. — The palliai sinuses increase in length, but 

 they present no noticable changes in form. Even at the 15 p. c. 

 stage the posterior branches, (Nebenstämme) have not as yet made 

 their appearance. 



Blood corpuscles. — By this stage the blood corpuscles have 

 greatly increased in number (PI. VI., Fig. 80). The spindle 

 bodies, 1 a curious modification of the ordinary blood corpuscles, 

 are now to be noted for the first time. Though few in number, 

 the spindle bodies occur in both peduncular and body cavities 

 (PL VIII., Fig. 129, sp. b.). In length they measure 10-12/* 



1. Vide N. Yatsu :— Notes on the histology of Limjula, etc. Art. 5, This Vol. 



