19 



PLATE X. 



FRESH-WATER POLYPE. 



Genrral Characters — 



Figs. 1, 2. Foot or liydroihizn (Gr. rhiza, root) attached to some floating body. 



Mouth at free end. 



Tentacles or feelers (L. tentare, to feel) surrounding the mouth. 



^ , . f Testis, the male ori/an. 



Keproductive organs -! ^ i^ e ^ 



"^ ( Uvary, the temale organ. 



Multiplication by budding or gemmation (tig. 2). 

 General Structuee — 



DiAG. 1. Ectoderm (Gr. ehfos, outside; derma, skin) (red). 

 Endoderm (Gr. endoa, within). 



Body-cavity, serving also as a digestive cavity, and nniitinued into the tentacles. 

 Minute Structure or Histology — 



Fig. 3. Ectoderm-cells with contractile prolongations — neuro-niuscular cell-layer. 



Nuclei and nematocysts in the cells. 

 Fig. 4. Endoderm-cell with cilium on its inner surface. 



Figs. 5, 6. Thread-cells or nematocysts (Gr. nema, thread ; kwi/ns, a bag), consisting of capsule, 

 filament, and thi-ee recurved spines at base of thread. 

 Reprodcctiox or Sexual Process — • 



Fig. 7, a. Spermatozoon (Gr. sperma, seed; zoon, an animal) witli oval head and vibralile eiliuni. 



h. Eipe ovum, consisting of vitellus or yolk, germinal vesicle or nucleus, and germinal spot or 



nucleolus. 

 c. Impregnated ovum divided into a number of cells, surrounded by a capsule — burst to show 

 the contents. 

 Multiplication or Asexual Process — 



Process of gemmation shown in fig. 2. 

 Development of Hydrozoa — 



DiAG. 2. The impregnated ovum by division becomes a mulberry-like mass or monda, the morula 

 becomes hollow by the accumulation of fluid in its interior, forming a planuhi, and the 

 plauula by gradual thinning opens at one end, becoming the so-called gaslrula, which 

 passes into the mature form. 

 Note. — In the Hydra itself there is no proper plauula stage, as the two-layered form represented iu c has no cilia. 

 Classification — 



Sub-Kingdom — Cwlenterata (Gr. Jcoilos, hollow ; enteron, an intestine), because it is composed 

 of two cell-layers — ectoderm and endoderm — and has no cavity separate from the body- 

 cavity. 

 Natural Order — Hydrozoa, because a digestive sac is not marked off from the body-cavity, and 



the reproductive organs are external. 

 Genus — Hydra. 

 Common Name — Fresh-water Polype (Gr. pohis, many ; potis, a foot). 



THE EARTH-WORM. 



External Characters — 



Fig. 1. The segmented body with a mouth at one end, an anus at the other, and a swollen portion called 



the cingulum (L. a girdle) or clitellum (L. cliiellie, a saddle). 

 Fig. 2. Jlouth situated on the 2nd segment. 



External and internal pair of setse (L. seta, bristle). 



Openings of segmental organs, one on each side of all the segments except the two first. 



Openings of spermathe'cre or receptacles of the spermatozoa (f!r. i<perma, seed; titel-e, a repository). 



Openings of vasa dcferentia (L. excretory ducts) on ventral face of loth segment 



Openings of oviduct on ventral face of Iftli segment. 



