214 
A PARASITIC COPEPOD BELONGING TO THE GENUS 
MEDESICASTE (KR0YER), AND ITS RELATION TO 
THE TUMOURS IT PRODUCES ON THE FISH, TRIGLA 
GURNARDUS. 
By W. HAROLD LEIGH-SHARPE, M.Sc. (Lond.). 
(With 7 Text-figures.) 
Habitat and record. The genus Medesicaste (Kroyer), belonging to the 
family Chondracanthidae, is usually considered to be represented by but one 
British species, Medesicaste asellinum (Linn.) 1 , whose characters are diagnosed 
as very variable. It is open to question whether the additional appendicular 
details given herein and the presence of the conjunctive tubes (vide infra) 
are sufficient to warrant the erection of a new species. 
The following observations were made upon eight specimens of Medesi¬ 
caste $ taken from various Trigla gurnardus caught at Lowestoft in August, 
1918. They were, without exception, upon the gills, upon which they cause 
characteristic tumours. 
Body. The outline of the animal is best seen from Figs. 1 and 2. The 
cephalothorax consists of a subglobose anterior end or head, bearing the 
antennules and antennae, and a considerably elongated slender neck at the 
base of which are situated the mouth and the other cephalic and thoracic 
appendages, which are thus widely separated from the antennae. 
The head is 1*4 mm. long, bulbous, trilobate, rounded in front, with each 
side expanded into a hemispherical lobe. The neck , which has its posterior 
portion enlarged, is 1*8 mm. long. Both head and neck as far as the mouth 
are embedded in the tumour which is formed upon the gill-filaments of the 
host. 
The trunk is square in outline, 2*4 mm. long, slightly wider than long. 
It is divided by a deep transverse constriction into two almost equal parts; 
the anterior constituting the thorax bearing appendages, carrying two pairs 
of obtuse processes ventrally and a small pair dorsally; the posterior, the 
larger, the genital segment, the antero-lateral corners of which extend out¬ 
wards into angular projections. The lateral margins of this posterior division 
are arcuate, converging distally, and forming two small postero-lateral lobes 
between which is a small medio-ventral V-shaped depression or sinus. 
1 A full list of references and synonyms is given by Scott and Scott (1913). The British Parasitic 
Copepoda, The Ray Society, London. The only fairly good figure is that in this work, Plate LII, 
fig. 6. 
