THE SAND-EEL FAMILY. 313 



life-cycle and assume a littoral habit at once. This subject is 

 again referred to in the part upon the Herring, in which species 

 it will be seen that the autumn larvss do effect this short cut, 

 eliminating the pelagic stage from their life-history. 



It has been stated above that the lesser sand-eel probably 

 has two spawning-periods in a year, and facts have been quoted 

 in support of this statement, derived partly from the occurrence 

 of larval forms and partly from the examinabion of adults. This 

 result will not be invalidated by the occasional occurrence of 

 isolated examples of spawning adults or of larvae in the inter- 

 vening period, for an extended spawning-habit is probably the 

 precursor of the condition in which spawning is affected at two 

 different seasons of the year. Experience teaches us that a 

 ' spawning-period ' is merely that duration of time in which 

 the great majority of the individuals of a given species 

 fulfil reproductive functions. 



We may briefly summarise the life-history of the young 

 sand-eel as follows : — 



A sand-eel from an egg spawned in December or January, 

 after a qiiiescent period imbedded in the sand, emerges upon 

 the surface of the latter at a length of about 4 to 5 mm. At 

 this stage the supply of yolk-material is generally exhausted, 

 and the oil-globule only remains. The post-larval sand-eel 

 keeps to the bottom till a length of about 10 mm. is reached. 

 Living in company with it are found great numbers of larval and 

 post-larval herrings, usually somewhat larger, and young arrow- 

 worms (Sagittce). There is a remarkable superficial resemblance, 

 caused solely by the attenuated form of these three very diverse 

 organisms, which is, of course, entirely absent in the adult stages. 

 At a length of somewhat over 10 mm. the young sand-eel com- 

 mences its migration upwards through the mid-water, and at 

 this period its growth is very rapid. The average date for 

 this change of habit will be seen to be about the end of April 

 (see Diagram), sometimes earlier. By about the third week 

 in May, or sometimes sooner, the surface will be reached, and in 

 the three weeks' migration through the mid- water the little fish 

 will have grown from 10 — 11 mm. to 17 — 18 mm., a very rapid 

 rate of growth. The surface-period lasts from 17 mm. to about 



