90 LECTURE V. 



become aggregated in the centre of the locomotive larva, which may 

 be compared to the locomotive germ or zoospore of an alga or sponge. 

 It is, in fact, a single-celled animal with a ciliated as well as a con- 

 tractile cell-wall. 



In a short time after it has escaped from the ovum the ciliated 

 integument ruptures and its contents disappear, with the exception 

 of the concentrated nuclear matter, which is left clear and distinct, 

 and of a definite form. A spontaneous movement is observed in this 

 body : it grows, and now a granular structure may be seen in it 

 under a high magnifying power. Before its escape it seems to be a 

 compact structureless mass ; but afterwards numerous points or 

 centres of independent force begin to operate, and give rise to multi- 

 tudinous minute granules or nuclei, and it takes on a structure com- 

 parable to that of the Gregarina. Now, what is the condition of this 

 second phase or form in the metagenetic progress of the entozoon ? 



It will be observed that the embryo of the monostoma, when it 

 quits the ovum, is not like the chick ; the primary germ-cells have 

 not been converted into numerous and diversified tissues and organs ; 

 the great majority of them remain unchanged, and without exhaustion 

 of the spermatic force. This force would seem to be concentrated in 

 the clear nucleus, which expands to constitute the smooth-skinned 

 Gregariniform worm. In this, as in the pseudo-navicellar capsules, 

 numerous unchanged germ-cells or nuclei set up as many centres of 

 development, from each of which a cercariform embryo results. In 

 this process many of the germ- cells and nuclei are metamorphosed 

 into organs, and a corresponding proportion of the spermatic force is 

 exhausted. What remains serves to govern the subsequent develop- 

 ments, which result in the change of the individual Cercaria into a 

 Monostoma or a Distoma ; this is a " metamorphosis ; " but the ante- 

 cedent phases should rather be called a " metagenesis," * and the con- 

 ditions essential to that act are the retention of a due proportion of 

 the primitive germ-mass unchanged, and with its primarily received 

 spermatic force unexhausted. 



Some of the Trematode Entozoa are remarkable on account of the 

 places they are found in ; as, for instance, the Diplostoma volvens, 

 which infests the interior of the eye of the perch and other fishes. 

 The pupa of this species has been found in the eye of the perch, 

 coiled up, adhering to the inner side of the cornea, and then there 

 has been observed an oblique line, or trace in the cornea, which shows 

 how the cercariform larva bored its way through to get there. But 

 frequently it does not get so far, and one finds such pupa) in cases like 

 a watch-glass adhering to the skin or conjunctiva outside the eye. It is 

 probable that different stages of the Diplostoma volvens have been de- 



* XXX. 



