1 8 6 Phylum A nnelida 



does not take place until the spermatozoon enters the egg, the germinal 

 vesicle being indistinctly visible through the yolk. 



EMBRYOLOGY 



It is desirable to delay insemination of the eggs for half an hour after 

 spawning has occurred because the spermatozoa are quite immobile 

 when first extruded. Under the stimulus of seawater they gradually be- 

 come activated but are relatively inactive at best. After allowing time 

 for activation, remove the eggs by means of a pipette to a fresh dish of 

 seawater and add four or five drops of sperm. 



After fertilization the eggs develop within ten hours into actively 

 swimming gastrulae and therefore rise from the bottom. They may now 

 be poured into a clean dish, thus discarding the eggs which failed to 

 develop. 



Within 24 hours the embryos have become transparent trochophore 

 larvae which continue to swim actively. They remain in the trochophore 

 stage of development for 10 days or two weeks showing little external 

 change except a slight slender outgrowth at the posterior end which 

 constitutes the beginning of the worm body. They feed readily upon 

 diatoms by means of a ciliary mechanism and may be kept indefinitely 

 under laboratory conditions. 



Because they at no time settle to the bottom, it is difficult to keep the 

 water changed but they remain in good condition if poured daily into 

 clean dishes discarding the bottom layers. They have a tendency to 

 collect at one side of the dish and may be transferred to clean dishes of 

 seawater by means of a pipette, but this method involves the loss of 

 many embryos. Zeleny (1906) has reared them to metamorphosis in 

 aquarium jars. 



POST EMBRYONIC DEVELOPMENT 



The trochophore finally develops a slender worm body, settles per- 

 manently, and secretes a calcareous tube. By placing mollusk shells or 

 stones in a cage and sinking them in shallow water which is known to 

 contain breeding Hydroides worms it is possible to secure many young 

 worms and study their further development. Studies of this character 

 carried on at Woods Hole for several years have shown that they become 

 sexually mature in seven or eight weeks before they are half grown. 

 They become fully grown in two years. Most of the worms ordinarily 

 collected are only one year old and it is likely that many if not most of 

 them die during the second year. 



The shell of an average-sized worm after one year's growth, measures 

 65 or 70 mm. in length and 3 or 4 mm. in widest diameter. The largest 

 worms may reach 120 mm. in length and 5 mm. in greatest diameter. 



