SEASONS 



SEA-URCHINS 



285 



Seasons. In the article EARTH the motions 

 of the earth on which the changes of the seasons 

 ultimately depend are explained. The chief cause 

 of the greater heat of summer and cold of winter is 

 that the rays of the sun fall more obliquely on the 

 earth's surface in the latter season than in the 

 former (see CLIMATE). Another concurrent cause 

 is the greater length of the day in summer, and of 

 the night in winter. Within the tropics the sun's 

 rays have at no time so much obliquity as to make 

 one part of the year very sensibly colder than 

 another. But the zone of equatorial calms in which 

 rainfall is practically continuous is shifted north- 

 ward when the sun moves northward in the northern 

 summer, and is similarly shifted southward in the 

 southern summer. As tlie wet-zone swings to and 

 fro, following the sun, the regions it traverses 

 experience alternate wet and dry seasons. Those 

 regions lying near the mean position of the wet- 

 zone have thus two wet and two dry seasons in the 

 year, the regions near its extreme positions having 

 one wet and one dry season. Wet and dry seasons 

 are also produced l>y the Monsoons (q.v.), them- 

 selves due to the relative seasonal change of tem- 

 Iieratnre between land and sea (see also RAIN). 

 n the temperate regions of the glolie the year 

 is naturally divided into four seasons Sprinif, 

 Summer, Autumn, and Winter. In the arctic and 

 antarctic regions spring and autumn are very 

 brief, and the natural division of the ye.ir is simply 

 into summer anil winter, the winter being long, 

 and the summer short; and this is MTV much the 

 case also in regions of the temiwrate zones lying 

 near the arctic and antarctic circles. In subtropical 

 regions the distinction of four seasons is, in like 

 manner, very imperfectly marked. Conventionally 

 (as in almanacs) it is assumed that each season 

 commences at the equinox or solstice e.g. that 

 in the northern hemisphere spring commences at 

 the vernal equinox aliout March 20, and summer 

 at the summer solstice on June 21, although 

 this is popularly spoken of as '.Midsummer 

 Day;' and the 'summer months' in common 

 Knglisli parlance include May, June, and July, 

 winter l>eing November, December, and January, 

 and spring ami autumn accordingly. Practically 

 our division of the seasons depends more on 

 seedtime and harvest than on the extremes of 

 annual heat and cold. The greatest heat of 

 summer is reached a considerable time after the 

 summer solstice, the period when the sun's rays 

 are most nearly vertical, and the day is longest. 

 The greatest cold of winter in like manner occurs 

 after the winter solstice the period when the day 

 is shortest, and the sun's rays are most oblique. 

 The reason in the former case is that as 

 summer advances the earth itself IKJCOIIICS more 

 heated by the continued action of the sun's rays ; 

 in the latter, that it retains a port inn of the heat 

 which it has imbibed during summer, just as the 

 warmest part of the day is somewhat after mill-day, 

 and tbn coldest part of the night is towards morn- 

 ing. The four seasons of temperate regions are 

 dbtingnUhed by the phenomena of plant-life, such 

 us the budding, blossoming, fruit-l>earing, and leaf- 

 less repose of deciduous trees. Associated with 

 these annual changes there are modifications of 

 structure and function adapted to the seasonal 

 variation of climate, in different localities. Similar 

 habits of Hibernation (q.v.) or of change in the 

 tliiekness arid colour of fur or feathers are found 

 in the animals of regions where the seasons are 

 sharply contrasted in climate. The intellectual 

 superiority of the races inhabiting temperate 

 regions is in part traceable to the constant neces- 

 sity for forethought in providing for the regularly 

 recurring season of winter when natural resources 

 i to be available. 



Sea-spider. See PYCNOGONID^E ; and for the 

 Spider-crab, CKAB. 



Sea-squirt, a popular name for any of the 

 Ascidians (q.v.); also called Sea- perch, Sea-pear, 

 Sea- pork, &c. 



Sea-surgepns (Acronuridae), a family of 

 spiny-rayed Teleostean fishes, living in tropi- 

 cal seas, especially near coral-reefs. The name 

 refers especially to the members of the genus 

 Acanthurus characterised bv a lancet-like spine 

 which lies ensheathed on eacfi side of the tail, but 

 can be erected as a formidable weapon. 



Sea-swallow. See TERN. 



Seathwaite, a valley and hamlet at the head 

 of Horrowdale (q.v.) in Cumberland, remarkable 

 for its heavy rainfall. See RAIN. 



Sea-trout, a popular name for various species 

 of the genus Salmo, but especially for the common 

 Salmo trutta (see SALMON). 



Seattle ( pron. Se-at'tel), capital of King county, 

 Washington, is on Elliott Bay, an arm of Puget 

 Sound, 18 miles by rail N. of Tacoma. The resi- 

 dence streets run up the slope of a hill, with the 

 business portion built on the level ground at the 

 foot, stretching along the excellent harbour, with 

 its many wharves. Seattle owes its phenomenal 

 growth to the development of the state lumber 

 trade, of which it is the chief seat. Over 1000 

 vessels of about 1,000,000 tons enter and clear the 

 port annually, carrying away coal and timber. 

 Shipbuilding and a busy fishery are also carried on. 

 There are cable and electric tramways,, and the 

 town possesses a university. In 1889 a terrible fire 

 destroyed the whole business portion sixty blocks 

 witii the wharves, and cost nearly $10,000,000; 

 but within a year 265 new buildings, mostly of iron 

 and stone, besides sixty wharves, with a frontage 

 of 2 miles, were erected. Pop. ( 1880) 3533 ; ( 1890) 

 4'J,S37; ( HXKI) 80,671. 



Sea-unicorn. See NARWHAL. 



Sea-urchins (Echinoidea), a class of Echino- 

 derms. In the more typical genera, such as 

 Echinus, the body is symmetrical and nearly 

 globular ; others, such as Spatangus, are heart- 

 shaped ; and others, such as Clypeaster, are shield- 

 sliapcd and flattened. In all cases the body is 

 walled in by continuous plates of lime, which, 

 though cajiable of independent growth, are rigidly 

 connected, except in Echiiiotlinrida*, which have 

 plastic shells, as the extinct Paheo-echinoidea seem 

 also to have had. 



In a typical sea-urchin, such as Echinus escu- 

 lentus or" titrongylocentrotus lividus, the body is a 



Fig. 1. Common Sea-urchin (Echinus emulentus), one- 

 half with spines removed. 



slightly flattened sphere, covered with movable 

 spines. The food-canal begins in the middle of the 

 lower surface, and ends at the opposite pole in the 



