'i'O ZOOLOGY. 



The Siphonophores, as observed in Agalma, EfjihuKa, 

 Halistemma, and other forms, arise from eggs which pass 

 through a morula, planula, and gastrula stage. The further 

 development of Halistemma carmn, a Siphonophore native 

 to the shores of New England, has been described by A. 

 Agassiz as follows : In the earliest stage noticed the young 

 looked like an oblong oil-bubble, with a simple digestive 

 cavity. Soon between the oil-bubble and the cavity arise a 

 number of medusa-buds, though without any proboscis 

 (manubrium), since the medusa-buds are destined to form 

 the " swimming-bells," which take in and reject the water, 

 thus forcing the entire animal onward. After these swim- 

 ming-bells begin to form, these kinds of Hydra-like zooids 

 arise. In one set the Hydra is open-mouthed, and is, in 

 fact, a digestive tube ; its gastro-vascular cavity connecting 

 with that of the stem, and thus the food taken in is circu- 

 lated throughout the community. These are the so-called 

 " feeders." The second set of Hydras diifer only from the 

 feeders in having shorter tentacles twisted like a corkscrew. 

 In the third and last set of Hydras the mouth is closed, and 

 they differ from the others in having a single tentacle in- 

 stead of a cluster. Their function has not yet been clearly 

 explained. ISTew zooids grow out until a long chain of 

 them is formed, which moves gracefully through the water, 

 with the float uppermost. 



All the Hydroids in their free state as medusffi are more or 

 less phosphorescent, and as much or more so after death, 

 when their bodies become broken up, and the scattered frag- 

 ments light up the waves whenever the surface of the ocean 

 is agitated. From this cause the sea is especially phosphor- 

 escent in August and September, when the jelly-fishes are 

 dying and disintegrating. These creatures serve as food for 

 the whalebone whales, which swallow them by shoals. 



The smaller species are abundant in the circumpolar seas, 

 while in the tropics the Siphonophores are especially nu- 

 merous, none occurring in the Arctic regions. The Hy- 

 droids are widely distributed, a species of Campanularia be- 

 ing common to the Arctic and Antarctic seas. The species 

 occurring on the New England coast are in many cases 



