V CRUSTACEA— ONTOGENY 387 



of limbs is hatched from the egg. The shiekl-like dorsal integument forms on each 

 side anteriorly a process (frontal horn) at whose base glands emerge. There are 2 

 frontal filaments (frontal sensory organs) and an unpaired frontal eye. On the 

 under side of the head is a median projection of considerable size in the place of the 

 large upper lip of other Cirrqicde larvfe. At the posterior end of the body are 2 

 separate jointed caudal processes. Mouth, intestine, and anus are wanting. Near 

 the Nauplius eye is a cerebral ganglion. About the middle of the body lies a mass 

 of cells, the rudiments of the ovary. The JVauplms now moults 3 times and under- 

 goes during these processes a series of transformations preparatory to 



The Cypris Stage (Fig. 259, B), which it enters after its 4th moult. In this stage 

 we find a laterally compressed shell entirely enclosing the body, and consisting of 2 

 lateral valves which pass into each other, in the dorsal middle line, without articulat- 

 ing. The body consists of 3 regions, the large head, the trunk, and a rudimentary 

 terminal portion (the abdomen). The shell arises from the head. The head contains 

 the rudinient of the ovary, and carries the Nauplius eye, the frontal filaments and 

 one pair of antennse which have proceeded from the anterior uniramose pair of limbs 

 of the Nauplius. The two pairs of typical biramoae limbs of the Nauplius (2d 

 pair of antennse and pair of mandibular feet) have disappeared. The trunk con- 

 sists of 6 segments, which have formed during the last Nauplius stages behind the 

 head portion, and it has 6 pairs of typical biramose limbs which cause the swimming 

 movement of the larva. The short abdomen carries one pair of short appendages 

 provided with setse. Mouth, intestine, and anus are wanting. The larvfe still feed at 

 the expense of the nutritive yolk derived from the egg, which is thus gradually 

 absorbed. 



The Kentrogon Stage. — After a free life of at least 3 days the Cypris-like larva 

 fixes itself by means of one of its two antennse to the base of a seta on the back or 

 on a foot of a very young crab. It then throws off the whole tnink, so that only the 

 head is retained (Fig. 259, C, D). The organs retained in the head become indistinct 

 and, to a certain extent, fuse into a spherical mass which surrounds itself with 

 a new hollow cuticle under the old one. The shell is then thrown oiT, and another 

 new cuticle forms round the sac-shaped body within the old one, and in a crater- 

 like depression of this new cuticle a hollow arrow-like process is formed (Fig. 259, E). 

 The crater-like depression is then evaginated, the hollow arrow or borer is in this 

 way pushed forward through the antennse and pierces it and the soft chitinous cuticle 

 at the base of the seta of the host, and thus penetrates the body of the latter (Fig. 

 259, F). Through this hollow arrow the whole contents of the pouch now pass over 

 into the body cavity of the host, and after becoming surrounded with a new cuticle 

 are known as 



" SaccuUna Interna." — All the organs of the adult &(ccaZMi« are formed out of 

 the cell masses which have in this way passed over through the arrow. Among 

 others the testes are now first developed, and are thus, as compared with the ovaries, 

 very late. The SaccuUna interna lies on the abdominal intestine of the host, and feeds 

 by means of numerous root-like processes proceeding from its surface and penetrating 

 the viscera of the host. As the SaccuUna increases in size it exercises pressure on 

 the musculature and integument of the host, which die away on the under side of 

 the abdomen in the immediate neighbourhood of the parasite, thus allowing the latter 

 to pass out, while the roots (now proceeding from a stalk) remain inside the host. 



Sacoulina Externa. — The cloaca, till now closed, opens, and at its edge dwarf males 

 are always found ; these have been shown to be animals which have remained at the 

 Cypris-like larval stage, but can be distinguished from the female Cypris-like larvae 

 by the fact that they develop no arrow. 



The attached Cirripedia (Lepadidce and Balanidce) like the SaccuUna pass through 



