General Morphology. 



15 



Sacculina occurs on the Brachyura; it is fixed usually near the junction of thorax and 

 abdomen, and when the tail of the crab is held apposed to the thorax, as in the normal 

 position, the jnantle opening points forward toward the head of the host and is situated roughly 

 in the middle line. But the apparent symmetry of the body relatively to the host is spurious, 

 for the long axis of the mesentery runs on the morphologically right side of the crab (Text 

 fig. 6 mes) passing from ring of attachment to mantle-opening (ci), and the genital openings 



mes 



^Cl 



Text fig. 6. 



are disposed to right and left of the mesentery (only the left-hand openings being exposed in 

 Fig. 5 C), so that the axis of symmetry of the Sacculina is horizontal to that of the host. 

 This condition is easily derived from that in Parthenopea (Fig. 5B) by simply pulling the 

 mantle opening (op) posteriorly, and at the same time flattening the body laterally, a process 

 that might natnrally result from the position of the parasite nnder the tail of the Brachyurous host. 



The genus Heterosaccus differs from Sacculina merely in the fact of the mesentery being 

 confined to the region of the ring of attachment, and not stretching down to the mantle 

 opening; the rest of the organs are disposed exactly as in Sacculina. 



We now turn to the second group containing the genera Lernaeodiscus and Triangulns. 

 These are both parasitic on symmetrical Anomurous genera, namely Porcellana, Galathea and 

 Munida. In Lernaeodiscus the mesentery has the form of a broad hinge clasping the visceral 

 mass in the middle line, the acute angle of the hinge being pierced by the ring of attachment 

 (Piate 7 figs. 33 and 34). Regarding the mesenterial surface of the body as the dorsal surface, 



