Introduction to A7itmal Alorphology. 183 



assume the adult form. Some of these larvae have 

 been found retaining the Rhabditis form in the 

 mollusc Physa. Ascaris nigrovenosa lives as a her- 

 maphrodite form, often 12 mm. long, in the lung of 

 the brown frog, and produces ova. The young, bred 

 from these, enter the cloaca of frogs, and assume a 

 Rhabditis form, with short, pointed tails ; these be- 

 come free, dioecious, in mud or water, and produce 

 eggs, which emit Rhabditoid embryos, often in the 

 uterus, and then entering into frogs, again moult, 

 lose their teeth, and develop forms like the original 

 Ascaris. 



Passive wandering occurs in some forms ; thus 

 some Strongyli begin life in Fish, and end in fish- 

 eaters. Cucullanus elegans begins in Cyclops, and 

 with these enters the stomach of fishes. Active 

 wandering occurs in Trichina spiralis, &c. Some 

 forms have a boring cephalic spine. About 900 

 species are known. 



They are divisible into four sub-orders : — 



I. Rhabdophora — free, marine, slender, bristled; with 

 two rows of pre-anal cylindrical rods, whereby they creep. 

 Chaetosoma has 2-15 pair of straight rods ; males small, with 

 two spicula ; females with two spermathecae and pre-centrally 

 opening vagina. Rhabdogaster has many short, curved rods, 

 in the midst of which is the female orifice. This genus has 

 no cephalic hooks, while the former has movable hooks in a 

 double series. These appear to be the nearest allies of 

 Chaetognatha. 



Sub-order 2. Cystoopseadae — imperfectly known, aproc- 

 tous parasites (under the breast-shields of the sterlet), with 

 thread-like males and oval cystic females ( Wagner). 



Sub-order 3. Hypophalli — rod-like appendages none; 

 penis ventral, in front of the tail. This includes fourteen 

 families: — i. Cucullanidae — parasitic ; head distinct. Cucul- 



