September 1, 1891.] 



KNOWLEDGE. 



177 



Fie. 2. 



which protect the animal from getting rubbed off, as 

 it otherwise would, owing to the violent movements given 

 to the shell at times. By the gradual growth of the 

 ccenosarc, and the budding up at intervals, the colony 

 spreads over the shell. Besides the pohjiiitfs there spring 

 at times, more often on the rim of the shell, other 

 curious processes, which although they appear to be 

 of somewhat the same substance as the polyps, never 

 attain their form. They are like timi curred fihiments and 

 roll and unroll in a most curious and lively fashion ; they 

 are probably accessories to nutrition, and act as fishing 

 lines. 



The colony may go on increasing in this manner for 

 some time, till it almost covers 

 the object on which it grows ; 

 after a time, however, accord- 

 ing to conditions (generally in 

 the summer or autumn), many 

 of the poll/piles begin to show 

 little buds just below the 

 tentacles. They appear as an 

 enlargement of the ectoderm 

 or outer body of the polyp. 

 (Fig. 2). These are the yano- 

 pliuren which develop the motile 

 form or i/ono-ooid, whose office 

 is to found a future in- 

 dependent colony. This bud 

 gradually enlarges, till the 

 ectotheca or external covering 

 bursts, and the nunoziioid or 

 mednsa form (still attached to 

 the polypites by a narrow 

 stem) unfolds itself. (Fig. 3.) 

 An hour or so of pulsations 

 or contractions of the bell, 

 and the new body breaks from 

 the stem of the parent, and 

 dances away through the 

 water, a lovely translucent bell, 

 the minute structure of 

 which will be better seen in 

 Fig. 4. Here we have a delicate transparent bell, some- 

 what contracted at the opening, where the rim is boimded 

 by a sort of circular canal. From this at regular intervals 

 proceed eight arms or tentacles, very similar in structure 

 to the tentacles of the polypite, being very contractile and 

 furnished with neiiuitoci/stn or thread cells, which are 

 supposed to have a deadly poisonous and paralyzing action 

 on any living thing touched by them ; but tliis I think is 

 scarcely proved, as the irritating effects, well-known to 

 bathers, in the case of tlie larger medusre, may be caused 

 by the mechanical effects of the number of minute 

 threads shot out into the surface nerves of the skm. It has 

 also been constantly observed that small entermostracse, 

 after being caught and held some time by the ten- 

 tacula, get free and swim away apparently none the 

 worse. At the base or root of each tentacle will be 

 observed a semi-opaque granular mass ; this is called the 

 (icell.u.i, and is supposctl* to be an organ of sense. From 

 the apex, and lianging down into the cavity of the bell 

 (which we may here also observe is partially closed at the 

 bottom by a delicate horizontal membrane called the vellum ] 

 is a kind of " clapper," which is really the body of the gono- 

 zooid, and is called the iiKinuhriiiin, at the tip of whicli is the 

 mouth, furnished with four short fringed lips, which are 

 very sensitive, and constantly in motion. In the walls at 

 the side of the manubrium are developed the ova and 

 spermatozoa for the founding of a new colony of polyps, 



FlQ. 



and the life-history of the animal is complete. This may 

 be considered as a type of the ordinary history of the 

 Hydroidx. In general the polypite colony is the permanent 

 form, and the medusoid state the ephemeral one ; indeed, 

 one may compare the medusa form of the polyp to the 

 butterfly or the winged aphide, or to the flower on the 

 plant. To quote Van Beneden : " Dans les plantes comme 

 dans les animaux la vie est generalement longue et la 

 tenacite grande dans les individus agames, ephemere et 

 delicate, au contraire, dans les individus sexue's. L'analogie 

 entre la meduse et la fleur se confirme de jjIus en plus." 



I have taken this particular zoophyte as an illustra- 

 tion of others because it can be easily procured and kept 

 in the aquarium, but the student who wants to see this 

 marvellous life-history of the Hydroids will find abundant 

 variety of other forms. In some of them the gono- 

 zooid is of more complex and larger form, but on this 

 occasion I have only described what I have actually seen 

 myself. 



THE FACE OF THE SKY FOR SEPTEMBER. 



By Herbert Sadler, F.E.A.S. 



GROUPS and isolated sunspots are increasing in size 

 and frequency. The following are conveniently 

 observable minima of someA Igol type variables (ef. 

 " Face of the Sky " for July) : Algol. — September 

 3rd, 6h. 24m. p.m. ; September 20th, llh. 17m. 

 P.M. ; September 23rd, 8h. Cm. p.m. U Corona}.— Sep- 

 tember 4th, lOh. 31m. p.m.; September 11th, 8h. 14m. 

 P.M. U Ophiuchi. — September 1st, 7h. 40m. p.m. ; 

 September 6th, 8h. 3Gm. p.m. ; September 11th, 9h. 21m. 

 P.M. ; September lOth, lOh. 7m. p.m. ; September 21st, 

 lOh. 52m. P.M. ; September 27th, 7h. 4Gm. p.m. 



Only two planets can be really conveniently observed in 

 September, Venus being in superior conjunction with the 

 Sun on the 18th ; Mars, though rising on the last day of 

 the month two hours before the Sun, has too small u 

 diameter (4-0") to be of any interest to the amateur; 

 Saturn is in conjunction witli the Sun on the 13tli : and 

 Uranus and Neptune are too near the Sun, as evening and 

 morning stars respectively, to be observed with any profit. 



