326 MB. A. HAKCOCK ON THE ANATOMY 



or two before recommencing in the opposite direction. The pul- 

 sations in Botrylloides radiata are nearly as numerous as they are 

 in the last species. In one individual 102 beats were counted in 

 the one direction, and 115 in the other. 



The above account of the circulation will be found to agree with 

 Dr. Lister's description of it in Perophora, so far as it was deter- 

 mined in that form ; but that excellent observer did not detect 

 the flow of the blood through the suspenders, although "fila- 

 ments" attaching the branchial sac to the mantle are described 

 and figured by him. Their function as blood-carriers seems equally 

 to have escaped detection by Yan Beneden, though he must have 

 been aware of their existence as bands or ties ; for they were 

 figured by Savigny*, who described them as ligaments attaching 

 the branchia to the inner tunic, and they are well known to ana- 

 tomists generally. Van Beneden, however, discovered the neces- 

 sity of a passage for the blood-current from the " periintestinal 

 cavity " to the branchia to prevent engorgement when the pulsa- 

 tions of the heart were continued for any length of time in one 

 direction. He therefore believed that the required communication 

 was efi"ected through the agency of the " respiratory tentacles " f. 



It will now, however, be of no avail to discuss the improbability 

 of such an opinion, since ample communications have been demon- 

 strated. But it may be remarked that these tentacles are un- 

 doubtedly hollow, and that in each there is a double channel, that 

 the blood will assuredly pass up one of them and down the other, 

 and that it will oscillate in unison with the movements of the 

 heart. In fact, Van Beneden states that he has seen it do so. I 

 have observed nothing to warrant the belief that either of the 

 channels is in immediate communication with the vascular net- 

 work of the branchial sac. On the contrary, they both seem to 

 me to open into the pallial plexus, which of course is continued 

 into the wall of the inhalant tube. 



The blood-system does not appear to vary much in the Tunicata : 

 though certainly I have not traced it in the other genera so com- 

 pletely as in Ascidia, yet enough has been seen to warrant the 

 above assertion. The heart is very similar throughout all the va- 

 rious forms examined ; but its position is not by any means con- 

 stant. In Ascidia parallelogramma it is placed on the anterior 

 margin of the stomach, and in connexion with the left side of the 

 mantle or inner tunic, following the removal, in this instance, of 



* M^raoires sur les Animaux sans Vertebres, pt. ii. 

 t Op. cit. p. 113. 



