1919 Fasten; on Salmincola 159 



tive tract from the body wall of the organism. This arrangement can be 

 seen in Figs. 19 and 20. 



/'. The nervous system is prominently developed in the first cope- 

 podid larva. The brain (Figs. 11 and 18, b) is a massive structure, con- 

 sisting of the supra- and infra-oesophageal ganglia united into a single 

 mass which completely surrounds the jjosterior portion of the oesophagus. 

 Fig. 18 is a transverse section of the organism in the region where the 

 brain (Fig. 18, b) is best developed, right below the eye (Fig. 18, e). 

 Anteriorly, the brain gives off slender nerves (Fig. 11, b) to the antennae 

 and the other mouth parts, while dorsally it gives off the optic nerve of 

 the eye. Posteriorly a prominent nerve cord, which appears to be single 

 (Fig. 19, n), makes its way from the infra-oesophageal ganglion along the 

 ventral surface of the stomach. Whether this ventral nerve cord later 

 divides and gives off nerves to the swimming feet and abdomen could not 

 be determined. 



g. The reproductive organs have not yet developed in the first cope- 

 podid larvae, and therefore it is impossible to distinguish males from fe- 

 males. When sections of these larvae are examined, however, one can 

 always see a large, undifferentiated mass of germinal mesoderm along the 

 ventral and lateral margins of the thorax (Figs. 11, 19 and 20, g.ni). 

 This mass later produces the various portions of the reproductive organs. 



4. ATTACHED STAGES OF COPEPOD 



The first copepodid larva is constantly swimming about in search of 

 a host to which to attach itself. The organism is specific in its choice, only 

 attaching itself to the brook trout, Salvelinus fontinalis ]\Iitchill. When 

 it comes in contact with the right host, attachment occurs. In a previous 

 publication by the writer (Fasten, 1913) this was described as follows: 

 "By the aid of the microscope I observed the process of attachment four 

 times. As soon as the copepod comes in contact with the filament of the 

 gill, its mouth parts are inserted into the flesh, and by means of the pow- 

 erful claw-like second maxillae it begins to rasp the filament until it forms 

 a cavity within it. As soon as this occurs, the anterior portion of the 

 copepod's head, the frontal margin, is brought in contact witli the cavity 

 and the enclosed attachment filament is injected into the hole. The sjjher- 

 ical mushroom body adheres to the flesh and the regenerating tissue of the 

 gill soon encloses it tightly, thereby fastening the organism firmly. The 

 mouth parts are then withdrawn from the flesh of the gill filament. In this 

 condition the parasite remains attached for a short time. Then the second 

 maxillae detach the posterior region of the attachment filament from the 

 head margin and they themselves become perninmiitly attaclied to this 



