TWO PARASITIC FORMS OF TUE TAMILT TETRAEHr:NCHlU^. 337 



proboscidis muscle) the presence of tlie bulb fibres would be ne- 

 cessary to withdraw the tendon, shifted gently upwards during 

 evolution, and so to give a moveable yet firm point from 

 which the upper fibres could operate. The bulbs are firmly fixed 

 by their lower end ; and as the process is better exemplified 

 in the succeeding parasitic form, the subject is there entered 

 upon. 



The parenchyma of the head has the same components as that 

 of the segments, only difii'erently arranged. Internal to the chi- 

 tinous skin are the circular fibres ; and succeeding these is the in- 

 organic stratum, much more sparsely amassed. The radiating 

 fibres are not collected into a network, but pass generally dis- 

 persed from the skin to the fibrous layer encircling tlie bulbs, and 

 are equally intercepted by the longitudinal fibres which extend 

 from the head throughout the entire colony. In the neck the 

 longitudinal water-vascular canals are clearly present (fig. 8, c), 

 only reduced in calibre ; but what becomes of them in the upper 

 part of the head I cannot state, not having been able, from the 

 limited number of parasites at my disposal, to accurately trace 

 them out. 



Ahothros carcharias. — In the description of this parasite I shall 

 mainly limit the anatomical details to those points in which it di- 

 verges from the former species. 



Fig. 15 gives an outline of the animal form, natural size — in which 

 are distinguishable the head, the segmented colony, and the appa- 

 rent junction between these. The head («) is in shape an elongated 

 ovoid slightly flattened from before backwards, ^^^ inch long by yf -g- 

 broad by y§^" thick, bevelled off towards the free end, from which 

 at the extreme point the proboscides emerge. A. few longitudinal 

 furrows are present ; otherwise its surface is uniform and smooth, 

 in colour a dark red, more intense towards the free end, where it 

 it is often black ; there are no fossettes or suckers. The extent 

 of the colony attached to the head in any one of the specimens 

 did not measure more than 2 inches in length, the delicate 

 striated band of zooids j^^ inch broad by -j-ojj- inch thick, having 

 suffered rupture in every instance ; yet at least 200 segments 

 were present. The milk-white colour of the colony strongly con- 

 trasted with the dark red head. At the apparent point of junc- 

 tion of the head and colony (fig. 15, h) there was no constricted 

 portion to call a neck ; but the upper end was invaginated in the 

 head to the extent of y'^V '^'^ '^^ inch, or, to put it in other language 



