22 BULLETIN OF THE LABORATORIES 



Figs. 2 and 3, of Plate VIII, show the form of the antennae of the 

 female and male in this stage, the former being .78 mm., the latter 

 .79 mm. long. In the male the palp is gone and the appendage is 

 considerably developed, the longer ramus still showing its primitive 

 form. Fig. 4 figures the fourth foot of a male ^ inches long. Fig. 

 5 shows the abdomen of a larger male, and Fig. 6, the stylets when 

 they are .78 mm. in length. Fig. i shows the form of the head of a 

 male some time prior to maturity, and Fig. 8 illustrates the external 

 genital organs of one side at the same period. The testis, only part 

 of which is shown, is a double chamber. The penis is paired and 

 each factor is spiny. A common muscle protrudes them both by lateral 

 pressure, while a muscle peculiar to each serves as a retractor. Fig. 

 7 illustrates the form of the male antenna, previous to the branching 

 of the so-called frontal organ. As to the homologies of the " frontal 

 organ," there is no doubt that in Chirocephalus, as stated many years 

 ago, it is an appendage of the base of the antenna and the develop- 

 ment shows it related in position and formation to the branchial sacs of 

 the other limbs. 



There is a curious dimorphism among these animals, as in cope- 

 pods. The males, under certain circumstances, become sexually 

 mature, while the antennae retain an undifferentiated character. The 

 antenna in this stage, has a short basal process and the remainder is 

 rather short and bifid at the tip, one of the branches bearing a sharp 

 spine. The frontal appendage is simple, coiled, and regularly crenu- 

 late in this stage. The next stage is characterized by the elongation 

 of one of the short branches at the end of the male antennae and the 

 modification of the frontal organs. 



Explanation of Plates. 



Plate V. • 



Fig. I. Young nauplius larva of Chirocephalus. 



Fig. 2. The same in an advanced stage. 



Fig. 3. End of body of individual of same age as figure I, showing cavity 

 about the rectum and its muscles, the cells springing from the walls of the rectum, 

 the blood corpuscles, the valve at the posterior end of the stomach, and the prim- 

 itive segments outlined in the cellular mass. 



Fig. 4. End of the longer branch of the second antennae, with massed blood 

 corpuscles. 



Fig. 5. Mandible and palpus. 



