396 DR W. CARMICHAEL M'lNTOSH ON THE STRUCTURE OF THE 



sections. I have not been able to see any blood-vessel in the tissues of the head 

 in Borlasia. A distended pale portion may often be noticed in the central line 

 between the snout and the ganglionic commissures, as if the animal had gulped 

 water by the aperture for the proboscis, so as to distend the channel, but this has 

 no connection with the circulatory system. Transverse section demonstrates 

 that there is no other channel in the snout in front of the ganglia than that just 

 referred to. 



In long pale species, such as Linens lactea* Mont. MS., the intervention of 

 an elongated region between the posterior end of the ganglia and the anterior 

 border of the oesophageal region renders a special modification of the circulatory 

 channels necessary. Accordingly, it is found that after the fluid collects in the 

 spaces in front of the alimentary organ, it is conveyed by two long channels for- 

 wards to the ganglia, where the same ending occurs as in the other species. 

 These channels seem to be simple elongations of the ordinary lacunae, and are 

 represented in transverse section in Plate XII. fig. 8 ; thus forming an inter- 

 mediate link between Borlasia olivacea and the still more elongated post- 

 ganglionic region in Cephalothrix. 



In Meckelia annulata there are two great longitudinal vascular trunks (Plate 

 XIV. fig. 11, r), which lie within the inner or longitudinal muscular coat 

 opposite the nerve-trunks, and they are peculiar on account of their large size 

 and the granular nature of their contained fluid. They form a coarse network 

 in the oesophageal region as in Borlasia, and are continued forwards just within 

 the border of the snout to meet in a vascular arch. 



Whatever special function the oesophageal region may perform in regard to 

 digestion, it is clear that the circulatory fluid bathing its outer wall is placed in 

 a favourable condition for oxygenation, as the mouth now and then must give 

 entrance and exit to sea- water, under the influence of the powerful ciliary cur- 

 rents caused by the entire surface of this division. Besides, it is evident that 

 during the varied actions of the oral aperture (e.g., during feeding) the circula- 

 tion would sometimes be much interfered with if such a rete mirabile did not 

 exist. 



In Cephalothrix I can only make out two great longitudinal vessels, whose 

 positions are seen in the transverse section (Plate X. fig. 3, r), viz., nearly oppo- 

 site the nerve-trunks (n), from which they are separated by the chief longitudi- 

 nal muscular coat. There is thus in this system also a deviation from the 

 ordinary Borlasian type. The size of the vessels is proportionally larger than in 

 the latter, and their transparent fluid contains a number of minute corpuscles. 

 In the living animal each lateral vessel may be observed to contract regularly 

 and swiftly from before backwards, sending a wave of fluid towards its posterior 



* T am indebted to Mr Parfitt for living specimens ot this species from Devonshire. 



