122 CRUSTACEA. 



sideration the fact that round the central entoderm-cell in Cctochilus there lie- 

 elements -which divide into ectodermal and entodermal elements, it appears- 

 possible that Urbanowicz has taken tliis process for the formation of the 

 mesenchyme. 



The later fate of the mesoderm in the Copepoda has not yet heeii 

 clearly made out. It appears, however, that its elements, in the 

 segments of the Ncmplius stage, become divided more irregularly,, 

 after the manner of a mesenchyme, and very soon become grouped 

 as the organs of the Nauplius. Certain cells come to lie along the 

 intestine, and give rise to its musculature, others form the muscles of 

 the limbs, or unite to form the antennal gland. The body-cavity 

 here exhibits the character of a pseudocoele. In the posterior part 

 of the body of the larva, which yields the remaining and greater 

 number of body-segments, on the contrary, a true paired mesoderm- 

 band is developed ; in this, according to Urbanowicz (No. 23) and 

 Fritsch (No. 20), the rudiments of true coelomic vesicles appear. 

 'J he most anterior pair of these vesicles represents the maxillary 

 segment. The dissepiments between the consecutive coelomic vesicles, 

 which Grobbbn also (No 21) appears to have observed in the 

 abdomen of CetocMhis, disappear in the later stages, whereas a 

 dorsal and a ventral mesentery are said to persist throughout life 

 (Fritsch). The dorsal mesentery is attached to the back by the 

 separation of its two halves, thus leaving a median dorsal sinus 

 Avhich must be regarded as a remnant of the blastocoele and as the 

 homologue of the cardiac cavity. This dorsal sinus is connected with 

 the anterior portion of the body-cavity, which develops as a pseudo- 

 coele. Even in early stages, when the mesoderm-band is still short, 

 one large cell can be distinguished from the rest ; this is the genital 

 cell, which develops on each side into the rudiment of the genital 

 glands. [Of. Hacker (App. to Lit., Copepoda, No. I.)] 



The food-yolk, in Cetochilus, is present in small quantities, and 

 is of little importance. In the eggs of the parasitic Copepoda, in 

 which it is plentiful, it aj^pears, according to Van Beneden, that the 

 cells of the entoderm at first migrate into the yolk and take it up 

 into themselves, thus bringing about the appearance of a secondary 

 segmentation of the yolk. At a later stage, however, the cells 

 again rise to the surface of the mass of food-yolk, there forming an 

 epithelium which becomes the wall of the mid-gut (Fig. 73 C, e7i, 

 p. 148). The latter thus finally surrounds the remains of the 

 food-yolk decreased by gradual absorption (cf. below the formation 

 of the mesenteron in the Cirripedia, pp. 126, 174). 



