120 NEW GYRODACTYLOID TREMATODES PROM AUSTRALIAN- FISHES, 



portion, with three long processes, the upper two pairs being the largest; while 

 the more dorsal part is developed into two pairs of short stout bosses. The hooks 

 articulate with the complex between the ventral pair of processes. The muscula- 

 ture of the hook apparatus is attached to these processes. A pair of very large 

 powerful muscles, lying on either side of the median chitin bar, are inserted on 

 the common base of the two large more dorsally and posteriorly situated of the 

 processes, while weaker muscles are attached to the smaller processes. Numerous 

 transverse muscles — excessive development of the outer circular muscle-layer-^are 

 inserted on the longitudinal supporting bar and add to the complexity of the 

 whole structure. A more weakly developed transverse muscle lies i mm ediately ui 

 front of the longitudinal chitin-bar and lodges a small sesamoid-like piece of 

 chitin which articulates with the base of the latter. 



Immediately internal to the crenated margin of the disc ai'e two rows of 

 numerous very minute hooks, each with a bifurcated basal attachment (PI. xxii., 

 flg. 84). 



The head develops two prominent head-lappets (characteristic of Calceostoma) 

 provided with the curious ornamentation seen in the figure. The fact that blood- 

 corpuscles from the host occur in the intestine of the parasite suggests that these 

 organs have a suctorial function; though it is also possible that an undulating 

 movement, for which they seem well adapted, would serve to waft food towards 

 the mouth. 



The integument (PL xsi., flg. 76) possesses a well developed cuticle, below 

 which lie the muscle-layer's with which it is connected by a rather loose sub- 

 cuticular tissue. The outer circular musculature is very poorly developed, and 

 in places quite absent. The longitudinal layer is strongly marked and interior 

 to it lies a second circular layer, much more prominent than the outer circular. 



In the posterior region of the animal, behind the intestine, lie masses of 

 unicellular glands which appear to open on the ventral surface. 



The mouth lies ventrally immediately behind the lappets and just in front of 

 the pharynx. The buccal cavity is short; the pharynx large, prominent, and de- 

 void of gland-cells within its walls. There is no definite oesophagus. The intes- 

 tine is bifurcated, the two limbs joining again behind the testis. Intestinal caeca 

 are well developed, especially on the outer side of the intestine, one pair extending 

 forwards half way along the pharynx. There is a similar posterior extension 

 considerably behind the connecting jDiece of the intestinal limbs. The intestine 

 is lined by a single layer of cells. 



In the anterior portion of the body are two great masses of unicellular 

 glands beginning considerably behind the phai-j'nx at the sides of the body and 

 gradually broadening out to form two prominent masses just in front of the 

 pharynx. From each cell a duct is given off (PL xxii., flg. 82) uniting with 

 ducts from neighbouring cells. In this way are formed a large number of trans- 

 verse ducts which run below the pharynx and appear to open into the buccal 

 cavity and, possibly also, into the ventral part of the pharynx. 



It is possible that these glands are homologous with the cephalic glands of 

 Gyrodactylidae , the characteristic head-organs of that family being probably an 

 accumulation of numerous ducts which are seen separated and distributed in 

 Dionchns where the arrangement seems to be intermediate between the two types 

 of structure. 



No other glandular organs could be seen in connection with the intestine. 



The excretory system can be observed in serial section lying immediately 

 below the intestine on each side (PL xxi., fig. 75) as a tube with a moderate 



