GASTRODES PARASTTTCUM KOROTNEFF. 75 



in the mantle of the host body (In my preh'minary paper, 1920, 

 b, I have given erroneously as if the parasite were found in the test of 

 Salpa). The parasite is situated with its ventral side commonly 

 directed towards the centre of the body of the host, but sometimes 

 away from that. As pointed out by Heider ('93), it may be found 

 practically anywhere in the mantle, and there is no tendency that it 

 inhabits more commonly in the neighbourhood of the' nucleus of the 

 host, contrary to the remark of Korotneff ('88). Tt may occur, as a 

 matter of fact, in parts far from the nucleus; even the processes 

 in front and behind of the body of Salpa are often infected by the 

 parasite. The frequency of the occurrence of the parasite fluctuates 

 within a fairly wide extent. In a swarm of Salpa which I met 

 with one day of 1-4, April 1919, only four individuals of Salpa out 

 of more than one thousand were inhabited each by one Gastrodes, 

 while, on another day of the same period, I could secure fifteen or 

 sixteen specimens of the parasite out of about one thousand individuals 

 of Salpa. And, judging from the examination of the specimens of 

 Salpa presented by Mr. Aoki, it seems that, the frequency of the 

 occurrence of the parasite may amount much higher — say, even up 

 to 25 percent of the number of the individuals of the host. Generally, 

 only a single individual is found in the same host, but, not very uncom- 

 monly more than one; in the case shown in the text-figure, the host 

 harbours four individuals of the parasite. It is usual in such cases as 

 mentioned last, that the parasites represent very different stages of 

 development. 



When it is liberated from the body of the host, the parasite 

 assumes as a rule somewhat more strongly vaulted appearance (fig. i). 

 It can alter its external shape only a little, it may become slightly 

 broader in some direction than other; it has, however, only a limited 

 capacity of changing its shape and has no power of locomotion whatever. 



Structurally, the animal shows fairly great individual variation 

 with difference of its size. In most individuals occur, at the mid-dorsal 

 point of the body, the sense-orgen (figs, i & 2, 5) and on the lateral 

 sides a pair of tentacular apparatuses {f). Besides, there may be found 

 in large specimens eight ribs {c), which radiate from the sense-organ 

 and run towards the margin of the body. 



In the large individuals such as represented in fig. r, which 

 measure about 3 mm. in diameter, the structure conforms largely with 

 that of ordinary cydippid ctenophores. The aboral sense-organ {s) 



