44 



Mechanical 

 control. 



Psychological 

 functions of the 

 other sensory 

 organs. 



Anal set*. 



parapodium bears a motor organ (the bunch of setas) and a number of 

 sensory organs (the soft filamentous cirri). No definite function 

 has, I believe, hitherto been assigned to these organs ; possibly they 

 generate the retiring habit, just as the bifurcators do in C. pnsio. 

 Amphicom sabella would be another good example. 



The Ammocharidae are a family of the Polychaeta which build 

 flexible sand-covered tubes lined with a transparent gelatinous substance 

 and are capable of burrowing slowly through the sandy sea-bottom. 

 The habits of Ammochares filiformis have been described minutely by 

 WATSON.* The thoracic segments, and the abdominal, except the last 

 three, bear each a pair of bundles of setae ; in the abdominal region 

 each bunch is followed by a torus bearing a large number of minute 

 uncini. We might be led to think that either the setas or the uncini 

 acted as the controlling organs of this animal, but this cannot be, for 

 WATSON has shown that the function of the bunches of setas is to 

 spread the cement from the thread glands when the animal is building 

 the internal gelatinous sheath, and that the function of the uncini is 

 to grasp the tube when the animal is boring through the sand. As 

 there are no other external organs that can have a psychological 

 function, there must be a mechanical means for retaining the larva 

 within its tube. This is accomplished by the transparent conical tip 

 of the elastic sheath, which has a small pore at the end, through 

 which the branchial tentacles can be passed and spread out, so that 

 the conical tip slips to the base of the tentacles and forms a close 

 fitting sphincter, which acts as a mechanical means of preventing 

 further egress of the tenant from its case. 



If the bifurcators of C. pnsio have such an important function to 

 perform, it is possible that the less remarkable hairs and setae may 

 contribute something towards building up the character of the larva. 

 The anal setae are found in all Chironomus larvas and are evidently 

 a characteristic of great antiquity. They are generally supposed to 

 be tactile organs to warn the larva of danger from the rear. In this 

 larva I doubt whether they are of any use whatever, in fact the larvae 

 in turning round in their tubes must find them a considerable 

 inconvenience ; while from the rear no danger is to be expected. They 



* Structure and Habits of the Polychseta, Linn. Soc. Journal Zool., vol. XXVIII., pp. 230-260. 



